47-55.1 Pressure bleeding

[72Mountaineer]

Using the Motive pressure bleeder on my brakes on the '53, MC/power unit/prop valve is below the floor, stock pedal. Not getting much pressure at all out of the back, front is better, can't really see any air being expelled at all. Rear line has 10lb valve, front has 2 lb, everything is new. The reason for the Motive is typical bleeding isn't working, pedal goes to the floor on hard braking and the thing just doesn't want to stop good. Front discs and S10, rears are 2nd gen Camaro drum, drums are adjusted OK. Is this a typical thing with below floor MC?

[mr48chev]

You do not want nor need more than about 15 psi of pressure on the bleeder . The old metal Amco pressure bleeder that I learned on in the high school auto shop in 1962 and then used for teaching from 1979 through 1991 was designed to operate with 15 lbs of pressure. You won't get the big squirt that you get with someone pumping the brakes but you are just pushing air out of the system and don't need that pressure.

The short version is that pressure bleeders don't work the brakes they just push air out.

[geezer#99]

Your statement of hard braking gets me to ask this.
Did you change the pedal ratio?

[mr48chev]

I'm taking it to believe that "hard Braking" means that he is running up the street and then laying on the brakes panic stop style.

Can you pump the pedal up after that hard stop if you pump it slowly several times? If the pedal comes up when you pump it a few times there is air somewhere. if it doesn't come up you have another issue.

Residual valves keep the pressure in the lines so there is pressure against the seals in the calipers and cups in the rear cylinders. You do have them installed in the correct direction ?
Is your free travel on your pedal adjusted correctly?

If the pushrod is adjusted too loose you may be running out of pedal travel just as you would if the drum brakes aren't adjusted. How much free travel do you have with the pedal? You want about 1/8 with the pedal all the way up

The late Dick Spadaro who was a well known vintage Ford guru gave these instructions for adjusting the pushrod.
Without explaining just what parts you purchased new that resulted in a different pedal experience makes trouble shooting difficult. Since you do have brakes but extensive pedal travel its more likely the problem one of these areas. If you have 1/4" free play in your pedal to master cylinder push rod you have way to much free travel. If your new pedal assembly pushes from the pedal to actuate the master cylinder via a clevis or heim adjuster you should re adjust the pushrod length.To get the free play necessary to prevent constant pedal pressure you should remove the bolt holding on the connecting clevis, pull the pedal to the up position desired. With the push rod inserted into the master cylinder and just touching the piston, adjust the push rod clevis so that the bolt hole opening in the clevis exposes 1/2 opening of the mounting bolt hole in the pedal arm on the master cylinder side.. Replace the clevis bolt and check the pedal to master cylinder free play, should be just a little 1/8" or so. Now just static brake check the distance the new pedal adjustment travels to the floor. If this is the same as before you have have not purchased a master cylinder with the same displacement as the old one as posted Mike 51 suggests. It is important to match the old displacement bore with the same displacement bore on the new master cylinder.

On the master cylinder bore he mentioned in that answer to a question the mc bore should be compatible with The bores of the calipers and wheel cylinders but I don't think that is the issue with your setup as those aftermarket brake setups usually closely match the metric GM calipers and matching rear wheel cylinders.
How did you adjust the rear brakes and how tight are they adjusted? I was taught to tighten the adjustment until you can't turn the wheel by hand and back the adjustment off 11 clicks. That works real well on the older axles with the adjustment slot in the backing plate but getting them right on brakes that don't have the slot is a challenge. My rule of thumb with that style is to take them out so I can just get the drum to slide over them and may have to help it a bit. Properly set, the shoes will have a bit of friction against the drums at all times.

I should note that in the 70's between the time I was in the Army and the time I started teaching I worked as a front end mechanic/ brake mechanic for several years and did a lot of brake jobs. Mostly on drum brake cars in those days. I had to deal with a lot of brake issues back then.

[dsraven]

q1
do you have a firm pedal or a spongy pedal
q2
what does your pedal linkage look like down to the master cylinder, are all the pivots and connections free moving but tight (no play) and the pedal isn't dragging on the floor seal
q3
do you have a pedal return spring and a pedal stopper at the top of the pedal stroke so the pedal has to stop at the same place every time it is released
q4
does your master cylinder have residual valves in the outlets. some rebuilt units made for disc/drum were previously used on drum/drum which had the master cyl res valves. a master made for disc/drum usually has one res bigger than the other because disc's have a bigger volume at the caliper than a drum wheel cylinder has so they need more volume in storage capacity to supply the system as the brakes wear and fluid is required. are the inline residual valves installed right way around
q 5
do the brake lines have coils , high spots etc that trap air
q6
is your linkage adjusted properly so when the pedal is at it's full released position and against the stopper there is free play on the pushrod at the brake booster end. this is required to allow the master cyl to return fully
q7
is the master cylinder to booster pushrod adjusted so the master cylinder has ability to return fully? just because the booster pushrod from the linkage has freeplay that doesn't mean the booster to master cyl pushrod is correct
q8
have you tried disconnecting a line at the master cylinder outlet and using your pressure bleeder to bleed that line to see if it is a line/system problem downstream of the master or a master cyl/booster/linkage problem? if the line bleeds easily then the problem may be that the master cylinder is simply not returning fully so the ports that allow fluid into the piston bore are only partially open to the reservoir. this is a big reason why I asked q6. lots of guys don't think a pedal stopper or return spring is needed but the weight of the pedal and linkage can easily overcome the spring at the booster so the master cylinder/booster sit in a position like somebody's foot is still on the pedal a little bit. a stopper also allows you to make the linkage free play adjustment at the booster with the pedal at the fully released position and you know that position will not change when the vehicle bumps over roads or the body twists up under load etc. the same reason a pedal return spring is also needed, to keep the pedal fully against the bump stop. if doing the bleed as suggested above and you get one line that bleeds easy and the other line doesn't then look at the prop valve, it may be shuttled if it has a metering valve integral. read on for that link
q9
do you have the park brake cables connected and adjusted properly. the park brake cables are an integral part of the rear drum brake adjustment and if not installed and adjusted properly then the system uses more fluid than normal (and thus pedal travel) to move the brakes shoes out against the drums before braking can start. the star wheel adjusts one end of the brake shoes to fit the drum and the park brake adjustment adjusts the other end of the shoes, at the wheel cylinder end, to fit the drum. otherwise the brake springs will collapse the wheel cylinders back into their bores when the brakes are released and then when you apply the brakes the first part of the pedal stroke is taken up moving fluid to those wheel cylinders to push them out until the shoes contact the drums. back in the day (well, even now) guys wouldn't bother using their park brakes or they wouldn't replace a broken or siezed cable. those were the guys that used a rock for a park brake or relied on that little park pin in the auto trans to hold their car when parked. they were also the guys that had to pump the brakes because the brake shoes collapsed back to their stoppers/anchors and it took the first brake pump to get them back out against the drums again.they would say "the brakes work fine. you just gotta pump them a bit".
q10
can you post some pics of your system
q11
is your master cylinder and prop valve all plumbed correctly to the brake system? like which port is the front brakes and which is the rear brakes? disc/drum master cylinders usually have the larger reservoir for the disc brakes so ensure that is set up correctly first. is the prop valve a combination valve that also has a built in metering valve? if so, is the metering valve portion centered? if it has the metering valve integral and the brakes were full of air the metering valve may have shuttled when you opened a bleeder screw and fluid started moving in the system. this effectively blocks the other system in the brakes so you really only get fluid to one circuit. the metering valve section will need to be re-centered to allow the brakes to operate properly. there is a tool required to properly bleed those prop valves, it keeps the prop valve open. if the valve needs to be re-centered there is a procedure for that too
https://techtalk.mpbrakes.com/propor...-bleeding-tool
mr48 is correct, don't use too much pressure on the bleeder because brake fluid in your face doesn't feel all that good when the bleeder is cracked, it takes paint off so if it gets sprayed on the paint and you don't notice it's to late, it also takes sealer off the concrete floor and dries the skin on your hands (or maybe thats the hand cleaner used to get the brake fluid off, haha). anyway, if it won't bleed with 15lbs then it sounds like your master cylinder is not fully returning or the prop valve has shuttled. re-center the prop valve and block it open and try disconnecting the pedal linkage and see if it bleeds then. if it does then adjust the pedal linkage as it may be holding the master from fully returning.
if you have a helper, and some safety glasses, and some room to see into the master cyl reservoir, have the helper step on the brake pedal while you look into the reservoir. you should see a couple of little fountains of fluid in the reservoir when the pedal is first stepped on. that is because the "feeder hole" for the bore is uncovered which allows fluid to fill the bore and also return to the res after a brake application or when the brakes heat up and the fluid expands. when you step on the brakes that very first little bit of pedal travel causes some fluid to be sqirted back into the res until the piston passes the feeder hole. if you don't see that little fountain or disturbance in the res fluid then possibly the master cyl is not fully retracting so the piston is partially covering that feeder hole to start with which would make it real hard to bleed the brakes and also cause brake operational issues. the piston in the bore needs to be able to retract fully when released so it sits behind that feeder hole. another reason for the pedal stopper and return spring, so the pedal stops at the same place every time it is released. figure out your pedal stopper and return spring, then figure out your pushrod adjustment between the master cylinder and booster so there is the right amount of free play, then figure out your linkage adjustment between the pedal and the booster so you have free play there. ensure you have the drum brakes adjusted properly with operational park brakes, ensure your brake lines don't have air traps in them, check to ensure you aren't doubling up on residual valves, ensure your prop valve is centered, then try bleeding the brakes. if you think the master to booster pushrod is possibly the culprit, and you just wanna do a quick check without removing the whole thing, you can try installing a flat washer on each mounting stud for the master cyl. between the master and the booster. this will give you some free play in that pushrod and if that fixes the problem then you know the pushrod is not adjusted properly

drum brake adjustment, assuming you are starting with clean parts that aren't gummed up with brake dust, mud etc and all the parts are operation, the backing plates do not have grooves where the brake shoes rub, all the springs are installed where they need to be, the star wheel adjuster is free to turn and lubricated, the drums aren't grooved out, the wheel cylinders operate smoothly etc etc. ensure you have the wheels on and tightened up as this makes sure the drums are fully seated on the axle flanges
-loosen park brake adjustment fully. this allows the wheel cylinder end of the shoes to fully retract against the anchor
-tighten the star wheel fully. not just until the brakes drag a little as that doesn't mean the shoes are actually centered in the drums. crank the star wheel until you physically can't turn the wheel
-back the star wheel off until the brakes drag just a slight bit, the tire is easy to turn. now the one end of the shoes is adjusted to fit the drum diameter
-now adjust the park brake. this will adjust the other end of the brake shoes to fit the drum diameter. now when the brakes are pushed the wheel cylinders will be pushed out and held at that spot when released. they will take less fluid on the next application so the pedal stroke will be less. no more "pedal goes to floor"
-understand that if you have new brake shoes and drums it doesn't mean they have the same diameter and will fit perfectly right away. that is the reason the park brake seems to need adjustment not long after your new brake job. the first bit of driving wears the shoe friction material to fit the drum diameter. after doing a brake job on a vehicle with drum brakes if you find the brake pedal height changes a bit you may need to do a few stops in reverse so the auto adjusters will do their job, then adjust the park brake again. if you had the drums machined then they are a larger diameter than the shoes so unless you bought the oversized shoes and had them re-arched to fit the drums ( not a lot of shops can even do that anymore, they just sell oversized shoes or new drums and regular shoes) then it may take some time to wear the new shoes to fit the oversized drums. dunno if that makes sense to you?
rear brakes should look like this. ensure the longer lining is in the right spot, the star wheel is the right way around so the auto adjuster works, etc
https://www.classicmuscle.com/p-75-8...hields-dbk75r2

this shows the residual valve locations in the master cylinder along with a few other topics and diagrams of how stuff works
https://slidetodoc.com/chapter-29-hy...brake-systems/

hope you get it working. let us know if you need more support.

[dsraven]

wow, sorry. that was a small book.
hope it helps somebody

[dsraven]

oh, one other question.
q12
does a full pedal stroke result in a full stroke of the master cylinder

[dsraven]

like mr48 says, those newer style drum brakes cheaped out on the inspection and adjustment holes on the backing plates. what I have done on lots of cars is to check where the holes need to be and simply drill a few holes and elongate them to fit the rubber brake adjustment hole plugs available at the parts store for cheap. I make a hole for the adjuster and also one to check the amount of lining left on the rear shoe, it's the one that wears fastest. some backing plates have a mark where the holes should be so it makes it easy to figure out. just be careful where you place the lining check hole so you're not in a spot where the shoes sit against the backing plate

[mr48chev]

That would be a simple fix that I should have already thought of.

I used to have two "special tools" pieces of 1/8 welding rod one with a loop on one end bent as a handle with a straight end on the other and one that had a little hook on the other end to push or pull the adjuster lever away from the star wheel .

[dsraven]

Haha, too funny. I have the same thing made out of a couple of hooks from the v belt assortment rack in the shop I worked at. Also the l shaped tools for opening the vent windows from the outside on the Chevy trucks. One pulls the lock button while the other pushes the vent window lever open. Those labourers were always locking their keys inside. I still have those tools in a toolbox drawer somewhere. Nostalgia.

[mr48chev]

I learned that trick at the cost of 45.00 when one of my brothers locked his keys in his Ford Bronco. The locksmith wanted to be paid first and then it took him 15 seconds to open the wing window.

[MiraclePieCo]

Originally Posted by 72Mountaineer

Using the Motive pressure bleeder on my brakes on the '53, MC/power unit/prop valve is below the floor, stock pedal. Not getting much pressure at all out of the back, front is better, can't really see any air being expelled at all. Rear line has 10lb valve, front has 2 lb, everything is new. The reason for the Motive is typical bleeding isn't working, pedal goes to the floor on hard braking and the thing just doesn't want to stop good. Front discs and S10, rears are 2nd gen Camaro drum, drums are adjusted OK. Is this a typical thing with below floor MC?

Did you bench bleed the MC?

And when bleeding did you use a low-pressure valve lockout tool like this below? MC low-pressure valves can easily deploy when bleeding.

.

[72Mountaineer]

I'm going to answer these Qs as best as I can. Pedal is spongy. Pedal linkage is free, stops at same place every time and return spring is great (as per dsraven in an other thread). As per Tuff Stuff (brand of MC/booster) theres a valve in the rear outlet, both in-line ones are correct. No coils or "peaks" in fluid lines. Plenty, probably 3/8", play in pedal pushrod to booster, booster to MC is as per mfgr. (about 1/32-1/16"). How would you do q8? (bleed each line separately). I do believe the pushrod is releasing fully, and should be pushing fully into the MC (x-ray vision is down at this time). E-brake seems also good, grabs when it should (not too early or late). When I bought the MC/Booster/Prop I talked to the Tuff Stuff rep and he recommended the correct combo for my setup. It is a Corvette-style MC suited for my disc/drum binders. I talked to the rep about the metering valve and he said it is where it needs to be when bleeding, you can see its position by a rubber-cap covered pin in the front of the prop valve. I've had the proverbial "brake fluid shower" from pumping the Motive tool up to 15 lbs. and have came to a 12 lb. good setting. Yes, hard braking means jamming them at 20MPH. Yes, I did bleed the MC before putting on, as per instructions. All in all, I'm doing the best I can with what I've got (knowledge and tools). I'm 68 years old, have done numerous brake jobs, replaced a few MCs, etc., so I'm not totally devoid of ability. That said, I still ask for help and/or advice, and this board is great for both. I've checked, rechecked, adjusted, etc. and have ran a quart of fluid through each the front and back wheels, and will still keep going/trying until I achieve what I need. Thanks to everyone.

[dsraven]

Do you think you are getting a full stroke on the master cylinder? Pedal goes to floor, with no air in system, (how could there be if you have bled that much) seems like not enough fluid is being displaced. All else being good.

[72Mountaineer]

If I wasn't getting a full stroke, wouldn't the farthest reservoir (which would be the front) suffer? I seem to be getting better flow through the front than the rear.

[dsraven]

curious, did you burnish or bed in the new brakes? burnishing does 3 things. it converts the materials/resins in the pads and rotors/drums to work better under the heat conditions they will see. it smooths the surfaces of the pads and rotors/drums by depositing a thin layer of the friction material onto the metal surfaces of the rotors/drums and it heat cycles the parts. I am sure you could find details on google. basically things heat up and theoretically changes the material. some of the friction material bonds/transfers to the surface of the rotors/drums and smooths the surfaces and makes for better friction. that's what brakes are all about, good friction.
when I burnish brakes I find a stretch of road that is going to let me do 60 mph for awhile. no lights, minimal traffic, high speed limit. the theory is to make several time spaced stops from 30-45 mph, this will allow the parts to heat up slowly and not thermally shock but also not overheat. then I make a few fairly aggressive stops. not enough to worry about skidding a wheel but enough to make smoke sometimes and also make me want to find a stretch of road where I won't get rear ended when doing this and also allow any vehicles behind me to pass safely. I don't want to make any full stops once I start the burnish process because I don't want the friction material to heat soak in a single position on a rotor/drum so I pick a section of highway etc with no stop lights for miles. I may notice the pedal seems to get spongy like brake fade. time to just do some driving now without having to use the brakes. this is to allow the parts to cool down evenly and also allow the resins n the friction materials to cool and cure. if I make a stop now the friction material will heat soak and thermally bond pad/shoe material to the rotors/drums in that contact spot and that makes for an uneven surface causing pulsating brakes later. the metal surfaces can also get like heat treated hard spots which will wear at a different rate than the rest of the surface, again causing pulsations. keep driving for long enough to allow a good cool down, this can be weather dependant. understand that the next brake application may seem like the brakes are on steroids. that is because some of the pad material has transferred onto the rotors/drums and that makes things kinda sticky. better friction.
a lot of friction materials (pads or shoe sets) will have instructions on how to break in, bed in or burnish.
here are a few

bendix
https://www.brakeandfrontend.com/tec...pads-or-shoes/

raybestos
https://www.motor.com/magazine-summa...eaking-brakes/

https://www.brakepartsinc.com/dam/jc...Burnishing.pdf

wagner
https://www.wagnerbrake.com/technica.../break-in.html

powerstop
https://www.powerstop.com/brake-pad-break-in-procedure/

they kinda all say the same thing. I am wondering if you have had a chance to do this step. if not done or not done correctly sometimes the parts will glaze and require excessive pedal pressure in order to stop the vehicle and/or the brakes can apply unevenly causing the vehicle to pull one way or the other.

[dsraven]

one more thing, and I am sure you have likely checked for this several times over, but are you sure there is no kinked, sharply bent or twisted lines?
what I meant by bleed each line separately, if you feel there is still air in the system, was disconnect the line that supplies, say, the rear. since it is the rear that seems to be giving the most problem? disconnect at the prop valve outlet so that eliminates another possible problem (prop valve or metering valve). you can buy a cheap plug to fill the hole left n the prop valve while you test the rear circuit. sort of like starting at the rear and working forward eliminating areas as you go. now do what it takes to connect the pressure bleeder to the rear circuit and bleed that out from the furthest away bleeder screw first. do you get air from the bleeders? do you get a full easy flow as expected with 12 psi? if so that eliminates the rear circuit from the prop valve back. next remove the plug where the rear brake line was connected (bleed tool installed in the metering valve as required for your system) and leave that port open. disconnect the rear brake supply line at the master cylinder and connect the power bleeder to that line. do you get proper flow at the outlet of the prop valve? air bubbles?
I am sure you can see where this is going for further tests. keep doing the circuits until you get the outcome you should and you end up finding the fault. just because parts are new doesn't mean they are flawless. once done you will need to bleed the system again but it should be easier next time if there were any flaws to fix
some pics of the system would help so we can see what you are dealing with. sometimes a new set of eyes will pick up on something overlooked.
you could also try a vacuum bleeder I suppose. they "suck" fluid from the system starting at the bleeder screw opposite of pressure bleeding.
was there an outcome from doing the visual of the reservoir while a buddy steps on the pedal?
3/8" of free play at the booster push rod may be excessive and could cause an incomplete stroke of the master cylinder I guess. still, you should only need 1/2 a pedal stroke for a full hard brake application. assuming the pedal ratio is correct for the booster/master set up

not implying that you are devoid of ability. just trying to help you get through diagnosing to get it working. it seems like there could be no possible way of air in the system if you have pressure bled a gallon of fluid through, still, I guess it could be possible. seems more possible that something is not quite right eiher spec wise or adjustment wise. maybe compare the bore size of your master with a stock S10 master for the year of your calipers or a stock camaro master for the year of your drums/shoes/wheel cylinder size
as for the question about getting full function from the front cylinder if not a full stroke. the master cylinder has a 2 part piston. think of it like the first piston to move is not mechanically connected to the second piston ahead of it, the second piston is applied when pressure is built up ahead of the first piston. this happens when the (in your case) rear brakes build pressure since your set up has the rear brakes connected to the rearmost port of the master cylinder-closest to the mounting flange. this built up pressure pushes the second piston in the master so it can start to move fluid that is ahead of it. if the rear system is blocked or restricted somehow then the fluid ahead of the first piston builds pressure almost right away and this pressure pushes on the second piston which moves in the bore and applies the front brakes. I dunno if that makes sense to you or not. maybe check this link or maybe you already know how the dual system master cylinder works. anyway, it may help a lurker if nothing else so see the link below

https://auto.howstuffworks.com/auto-...ster-brake.htm

one on how drum brakes operate

https://auto.howstuffworks.com/auto-...rum-brake2.htm

[dsraven]

I dunno if this makes sense but if you are not getting any feedback or resistance in the brake pedal until you are near the floor then it seems that the rear circuit is using a lot of travel to move fluid but not getting to the place where it builds pressure yet until late in the pedal stroke if at all, up to the point where the first piston in the master bottoms out against the second piston. like would happen if you blew a brake hose or some other failure in the rear circuit. most of the pedal travel gets taken up by that and then the second piston gets moving and you have front brakes pretty well at the end of your pedal travel. of course it seems like you have no brake action for the amount of pedal force applied because possibly the rear brakes are not doing much if anything.
maybe the next step is a flow and visual pressure test on the circuits to see what is happening. with the bleeder installed crack a line ahead of each rubber hose then do the same after each rubber hose to ensure the same flow is present with no reductions indicating a restriction and you can visually see if you think that fluid was under the 12 psi the bleeder is putting out. same for the res valves etc. work back to front eliminating possible problems as you go. even an overtightened brake fitting against a soft fitting seat can cause a deformed fitting flare or seat which becomes a restriction especially with the new style soft lines. I have seen new brake hoses that were factory overcrimped and caused a necked down inside diameter or even a new hose that was plugged in the rubber part
assuming
the system components are spec'd to work with each other as far as required output etc
the system is fully bled of air
the components are adjusted properly
the prop valve is operating properly allowing flow to both circuits
the metering valve is centered and operating properly to both circuits
the residual valves allow proper flow
no kinks or tight bends that would restrict fluid flow
no obstructed brake hoses
seems like next step is a flow and pressure test at each prop valve outlet then each bleed screw outlet
just trying to think logically about how the system should work and how best to diagnose. either start at the pump end (master cyl) checking output and flow and working downstream from there or starting at the outer ends and working back to the pump. there has to be a reason.
good on you for sticking to it. some would have given up.

[dsraven]

just thinking here, you did indicate the port closest to the mounting flange was connected to the rear brakes? did I get that right?
is the master cyl from a corvette with disc/drum? does it have a larger reservoir at the front-that being the one furthest from the booster? if equipped the large res should be for the disc circuit the smaller res for the drum circuit. if both reservoirs are the same size it could be a master from a disc/disc vehicle, connect the rears to the port closest to the booster. that way the rears apply first, controlled by the prop valve settings, then the front apply. that way the back end doesn't try to pass the front end when braking. you originally indicated there wasn't much pressure coming from the rear when pressure bleeding. possibly the prop valve is restricting? the master is faulty? spit balling here.

[mr48chev]

Good point. Are the lines connected to the master cylinder per the instructions that came with the kit.

Then one thing that came to mind while studying your photos on the brake return spring thread. Was that whole setup a matched kit from one vendor or is it something you have mixed and matched from various best deal vendors?

If you reach in from above or up from below with your hand and move the pedal arm how far does it move with just the gentle pull of your fingers before you feel resistance?

If you reach in and push the pedal with your hand with a ruler next to the pedal how far down does it go just with your hand until you feel resistance engine off and engine on?

I've got a stock set of pedals and mount out on my outside work bench and I'll try to remember to masure the length of the arm from the shaft to the hole for the bolt for the push rod in the morning as I think the stock one has a longer arm that gives a longer throw. I might be wrong but the arm on yours looks pretty short. Short arm if everything is perfect means a higher leverage factor though.

[mr48chev]

One thing we really need to have you do is write down a list of the exact parts you have either by part number or kit number.

Plus if you know the bore of the master cylinder that may solve an issue. While small bore master cylinders produce more psi they move less fluid and normally a power setup will have a slightly larger cylinder bore than a manual setup does.

[dsraven]

Curious what you have for s10 brakes. Earlier versions were single piston calipers and after 97 they were dual piston calipers. There could be displacement differences between the different types. The reason I wonder is so we could look up the appropriate year of s10 for the caliper style you have and see what size of master they ran. Compare to what you have. Also curious if that year of s10 also ran rear drums. If so what size of wheel cylinders did they run, and drum/shoe size, compared to your set up. The trucks are probably not far off in size/weight and brake bias. Really, if your rear drum brakes are the same as the s10, you could likely get away with running a stock disc/drum s10 setup complete with the prop/metering valve and some residual valves due to the under floor mounting. Just spitballing again and not suggesting you swap parts or start over. Just thinking of comparing specs to what the s10 had due to the same relative size and weight of the 2 trucks. See if your master is close to the s10 for specs.

[burnin oil]

Lots of good info and advice. One thing I would check is the wheel cylinders. Peel the rubber boot back and see if there is fluid in there. My 57 has a known leak and the symptom you describe is identical to mine. Decent brakes until hit hard. Bleeding was a pain also. Essentially the pressure was holding until higher pressures are applied then the leak would appear.
All the brake parts inside the drum were replaced before I got the truck.. Hope I didn't just confuse brake threads since I just read the new posts and this has been running a few days.

[mr48chev]

I'm pretty well positive that he has all new brake parts on the truck just as almost every mechanical part on the truck is new or rebuilt. I don't think that is the issue.

Brake adjustment= critical. on that axle if you don't have adjuster slots in the backing plates or drums you have to adjust the shoes out until you pretty well have to force the drums on over the shoes to have them tight enough. If you don't feel friction they aren't adjusted tight enough. Drum brakes are supposed to have a slight friction against the drum if they are adjusted right.

I'm still thinking that some adjustment isn't correct or some parts are mismatched for the application. Add that this is far from the first setup with aftermarket pedal assembly and tiny booster that I have seen guys with similar issues with.

[joedoh]

Originally Posted by 72Mountaineer

If I wasn't getting a full stroke, wouldn't the farthest reservoir (which would be the front) suffer? I seem to be getting better flow through the front than the rear.

just a note, on most GM the front reservoir is the REAR BRAKES. and when I say this, some people have shown me examples of like 50s corvettes that are front/front rear/rear but every s10 swap i have done with every booster/master i have reused the front reservoir is the REAR brakes. it has the larger hose which some people equate to the discs, but hydraulics dont really work that way.

so if you have a disc/drum master, this could be your whole problem. not enough pressure to the front.

just something to check, cant say thats what it is, but i would sure check.