brake question

[nvrdone]

my '49 3100 has the stock drum brakes at all 4 wheels. I decided to up grade & change the stock master cylinder to a mustang m/c with power booster. mostly to improve the stopping power and not worry about a complete brake failure if a line broke or wheel cylinder failed.
Big mistake. now the truck stops worse than it did before the change. I replaced all the lines with the same size as stock with 2 separate circuts. the kit came with an extension that needed to be welded to the end of the brake pedal short end to improve the ratio. I also added a 10# residual check valve into the front line.
Now the pedal goes within about 2" of the floor before there is any reaction and its scary to try and stop quick.
All the drums have been turned, new shoes installed and wheel cylinders replaced trying to solve the problem.
do I need to find a m/c with the same bore as stock ? any one have any ideas ? im about ready to install a stock m/c with out the booster.
thanks

[Foot Stomper]

Is the pedal spongey? Can you pump the pedal to get a firmer pedal? If so, this would indicate air in your lines... perhaps you've covered this already?

[dwcsr]

Originally Posted by nvrdone

my '49 3100 has the stock drum brakes at all 4 wheels. I decided to up grade & change the stock master cylinder to a mustang m/c with power booster. mostly to improve the stopping power and not worry about a complete brake failure if a line broke or wheel cylinder failed.
Big mistake. now the truck stops worse than it did before the change. I replaced all the lines with the same size as stock with 2 separate circuts. the kit came with an extension that needed to be welded to the end of the brake pedal short end to improve the ratio. I also added a 10# residual check valve into the front line.
Now the pedal goes within about 2" of the floor before there is any reaction and its scary to try and stop quick.
All the drums have been turned, new shoes installed and wheel cylinders replaced trying to solve the problem.
do I need to find a m/c with the same bore as stock ? any one have any ideas ? im about ready to install a stock m/c with out the booster.
thanks

Make sure you have the same diameter bore on the new master as you had on the old master

[nvrdone]

the system has been bled many times, both suction & back flow. the pedal isn't soft - it just goes down to about 2" off the floor before the brakes start to grab. pumping the pedal doesn't change anything.
I will check the new m/c bore. hopefully that may solve the problem
thanks for the ideas

[solidaxel]

You may need to add a 10# valve to the rear also if there is not one in the MC
Are the shoes adjusted up tight?

[Gippetto]

Sounds like it is a displacement issue. The master cylinder has too small a bore.

[yossarian19]

Originally Posted by Gippetto

Sounds like it is a displacement issue. The master cylinder has too small a bore.

Rockauto.com lists the half-ton trucks as having a 1" MC bore. Heavier trucks got 1 1/4" bore masters. A '68 mustang uses a 1" master, should work fine.

[dsraven]

just out of curiousity, is your truck drum front and rear? if so, the master cyl needs to match that. a disc front/drum rear master is gonna give you trouble. maybe check what the master was designed for.

a master bore that is smaller than the stock bore will not displace the same amount of fluid, thereby causing you to have more pedal travel to displace that fluid.the brakes take a predetermined amount of fluid so a smaller master bore means more pedal travel would be required if that is all that was changed. the power swap manufacturers give you a bracket to change the attachment point of the pushrod but that spot can only vary a certain amount before binding or misalignment occurs.

a power brake system will usually use a slightly larger bore size because a larger bore will make the brakes apply faster and the power unit can take advantage of the "boost" so less pedal effort is needed or stays roughly the same. larger bores take more pedal effort to get the same brake clamping effect at the wheels so the original design compensated for that with the pushrod attachment being closer to the pivot point of the pedal-for mechanical advantage- and possibly a longer travel in the master cyl which allows mopre fluid dispacement if needed.if you were driving a power brake car and the engine stopped while driving, you would have one brake application like normal-due to vacuum stored in the booster- then the brakes would seem worse than manual brakes because the larger bore master would take more pedal effore to achieve the same stopping power.

is your master frame mounted or firewall mounted? if frame mounted check all the cross shaft bushings and pushrod cranks etc for play that will take away pedal travel

if firewall mounted have a friend step on the brakes hard and see if the firewall is flexing, effectively taking away pedal travel

when that is all good:

check the adjustment of the pushrods in the system. there should be a small amount of play at each one, the link to the pedal and also the link to the master cyl. if too much free play then your pedal will be lower to start with because it doesn't actually contact the master cyl until lower down in the pedal stroke
check to ensure there is a return spring on the pedal so it comes all the way back to the stopper when the brakes are released. if it doesn't then the master cyl can't "top itself up" as it would normally at the end of each application. you can check this if you have a buddy step on the brakes while you look down (with safety glasses) inside the reservoir. when he steps on the pedal initially there should be a slight "fountain" or backflow of fluid into the reservoir just for a second until the piston passes the supply hole in the bore. this is because the piston returns back far enough to allow fluid to be returned to the res after an application, or allow expanded fluid to equalize on a hot day etc. if you don't get that fountain then maybe the master piston is not being fully returned to the end of the bore after a brake application.

check the pivot for the pedal and make sure it isn't worn and giving you extra pedal travel

if the drums were machined then they are actually oversize compared to original size shoes. the shoes will need to "wear in" or be re-arced to fit the larger diameter drums. if you took the drums off you would likely see the shoes are only worn in the middle because their diameter is meant for a smaller drum. an automotive machine shop I worked in way back had a machine that re-arced new, oversized, shoes to fit a machined brake drum. if the customer was buying new drums he got regular sized shoes. if he was maching the drums he got oversized shoes with thicker lining material and they got acred to fit the new drum diameter.

adjust the brakes with the park brake released and backed off. this is a step that a lot of guys overlook when doing brakes. the star wheel adjusts the shoes out to fit the drums on one end of the shoes-oposite the wheel cylinder- and the park brake adjusts them out on the other end-the wheel cylinder/anchor end. if the park brake is sloppy or non existant then the brake springs will bring the shoes all the way back in to butt up against the anchor, effectively bottoming your wheel cylinders out as well. then, when you step on the brakes, a whole bunch of fluid is taken up just to bring the shoes out to touch the drums again at the wheel cyl end of the shoes. that is why a lot of vehicles with drum brakes have to have the brakes pumped up to get a high pedal. don't laugh but lots of guys have no park brake and store a rock or block of wood as their "park brake". these guys usually also have to pump their brakes because the first application goes nearly to the floor in order to take up the slack down at the wheels.

start with a clean slate by backing off the park brake, adjusting the star wheels and rechecking after stepping on the brakes. I usually adjust the star wheels until the shoes are tight against the drums and then back them off the same number of clicks at each wheel to allow the wheels to turn freely. an extra screwdriver is required to push the star wheel ratchet back so the star can be turned backwards for release). when that is good then adjust the park brake.
how does the pedal feel now? any better? if not then maybe check into the bore size and master cyl overall length comparison

[_Ogre]

Originally Posted by dwcsr

Make sure you have the same diameter bore on the new master as you had on the old master

Originally Posted by Gippetto

Sounds like it is a displacement issue. The master cylinder has too small a bore.

with stock brakes you still need the same fluid volume
disc brakes typically have smaller bores thus less volume
you will probably need to change it out for a drum brake mc
or just upgrade to 4 wheel disc brakes

[nvrdone]

everyone: thanks for the input. after reading all the info, seems like ive done everything recommended. so it looks like the next step will be to change the m/c to a drum / drum one. any ideas on a part # ?

[mr48chev]

It may just be that the pedal ratio for the truck is that much different than the pedal ratio was on the Mustang.

Or that you have slack in the booster before the rod engages the booster the way it is now set up. I'd have a helper gently push the pedal while I was under the truck watching things move to see how far the lever pushing the rod and the rod move before the booster is engaged. It may just be an adjustment thing or needing to make an adjustable rod if the one you have isn't adjustable.

You might even be able to reach up with your hand and move the pedal lever to see how far things move before you actually start pushing fluid.

[dsraven]

have you checked to see that a full pedal application equals a full stroke on the master cylinder?

[nvrdone]

hey guys. Again thanks for all the feed back on this brake problem. I just took time to read thru the recommendations and as Mr 48 pointed out, I need to check to see if there is any slack between the booster and master. so how do I do that? I can check the slack between the pedal and the booster, but with the master mounted to the booster how do I check that slack ? thanks for all the help

[mr48chev]

There shouldn't be a problem there if you bought a matched set of booster and master cylinder. Where a guy would run into trouble there is picking up a booster because it was the right size and then getting a master cylinder because it bolted up to the booster but may not have been intended to bolt to that booster.

If you have ever looked at a number of master cylinders the hole or recess in the end of th piston where the rod goes can vary in depth for very similar looking master cylinders intended for different applications such as one intended for a manual brake setup being put on a booster that needs a piston with a shorter depth of the hole/recess.

I first had it happen to me back in the early 70's while doing a brake job and replacing the master cylinder. Solid pedal but it went down way too far before engaging the brakes. Pulled it back off took it back to the parts house and with some checking of other same number master cylinders we found out that the one I had had the wrong depth of hole for push rod even though everything else looked the same. I ran into the same thing a few years later on a friend's hot rod where he had mixed and matched parts. Everything bolted up but didn't match up. I think in that case he had put a manual brake master cylinder on a power booster on his rig.

About the only thing I can say on that is that the push rod sticking out of the booster should be long enough to go to the bottom of the hole in the piston of the master cylinder when you do some measuring. It's either yup it's real close or nope there is a serious difference.

[nvrdone]

ok, thanks for the info. I did buy the m/c & booster as a matched set, but you never know. when I get some time, I will pull them out & check.

[nvrdone]

ok, restarting an old thread. I finally had some time to start figuring out the brake problem on my '49.
The rear wheel cylinders have a 15/16" bore. any one know if this is the right size / too big / too small for stock rear brakes ?
cant find any accurate info on the internet.

[1project2many]

It looks like the 29-54 GM Parts manual is showing the rear cylinders at 1 3/16" from 36 to 50 and the fronts at 1 1/4". for the same year range. Casting nos on the original cylinders are 1074886 for th rear and 1074903 for the front.

http://chevy.oldcarmanualproject.com...929_54/296.htm

[solidaxel]

See #14 mr48 has a lot to say in the last paragraphs.

Did your master cylinder come attached to the booster when you bought it
If you had to assemble the two was there and resistance when you bolted up the M/C
Let us know if there was 1/8" </>

5/16 is a big difference on your wheel cylinders front

Also need to know the diameter of the M/C

Are all four the same size 15/16

[jweb]

I would add a residual valve to the rear before anything else.
Here's a good article about brake plumbing
http://mbmbrakes.com/typical-brake-s...onfigurations/

[ewalker302]

You said you replaced the wheel cylinders. Did you buy, or order the cylinders based on the year of the truck, or did you just get them based on the wheel cylinders that were on the truck.

Point is that if it had 7/8" cylinders on the rear, then it's a good chance that somewhere down the line, someone replaced the old style drum brakes with bendix style brakes that they started putting on these trucks in 53, or 54. Seems like it was a fairly common thing to do to the earlier models......i have a set of 1&3/16, and 1&1/4 old style cylinders that I ordered for my 50' that don't fit, because someone had already replaced my drums/brakes with the bendix style.

[dsraven]

ok, we have established that you have a firm pedal it is just at the floor by the time the brakes work. no air in the system.
next can you confirm that you have a park brake?
did you adjust the brakes properly with the park brake released and backed off, then adjust the park brake properly? this sets up the proper relationship of the brake shoes to the drums. without a park brake the wheel cylinders are pushed all the way back into their bores until the shoes butt against the anchors. it can take almost a full pedal just to bring the shoes out against the drums.
when you hit the brakes do they get better with a quick pump of the pedal? can the pedal be "pumped up"? once "up" does it stay firm and not slowly move towards the floor with the same pedal affort applied over a couple of minutes-simulating you stopped atr a traffic light? if it slowly creeps to the floor and there are no leaks it could be bypassing internally in the master cyl.
if it can be pumped up it could point to one or several of these areas to check
-your shoes are not adjusted right
-your park brake is not adjusted right or is missing entirely
-your pedal geometry doesn't allow a full stroke of the master cyl
-your pedal pushrod is not adjusted right to match the booster
-your booster to master cyl pushrod is not adjusted right
-your master cylinder isn't matched to the wheel cylinders size, so it doesn't displace enough fluid to move all the parts the required amount. you would need to know the wheel cyl size of the car it came from. your wheel cylinders are possibly requiring more fluid than the master cylinder was made to displace
-your master cylinder is made for disc front-drum rear and you have drum-drum


check it out and let us know. the wheel cylinders should be close to what the original master cylinder was designed for. larger wheel cylinders will obviously take more fluid to move the same amount and may also take more pedal effort to apply the same force due to hydraulic forces working on a different sized area of wheel cylinder pistons.maybe all you need is a master cylinder designed for a large car or truck that has the same size of wheel cylinders. get the pedal travel thing figured out and then work on proportioning valves etc for brake bias without skidding the rear wheels all the time. it can be hairy when the rear tries to pass the front on a hard brake application at highway speeds.

[nvrdone]

update: ok, I had some time to check out the brake system using all the suggestions made by you guys.
Front wheel cylinder - 1 1/8" dia. front brakes are stock bendix drum.
rear wheel cylinder - 15/16" dia. rear axle is 12 bolt out of a mid '60's c10 so rear brakes are stock to that truck
all brake lines were replaced & moved to left side of chassis before power M/C & booster were installed. system worked rather well when stock. up grade to power & dual circuit brakes was done more out of safety concerns. ( ever loose brakes on a single circuit system ? its scary!! brakes were / are adjusted to have a slight drag when wheel rotates.
nothing above has changed.

Installed power brake m/c & booster kit from CPP using the drum brake system as it was. I did add a 10# pressure valve in the front line there is no proportioning valve in the front line as I'm told you don't need one for a drum brake system. I extend the lower arm on the brake pedal that goes to the m/c per instructions in the kit. center to center is now
2 3/4". that should give better leverage. System has been vacuum & reverse pressure bled many times with no change.

Today I checked the push rods on the m/c & booster. there is no play in either push rod. Just for fun, I adjusted the m/c push rod out 1/8". at that point I got drag at the wheels indicating the m/c was starting to engage. I backed it off to its original length.
Brake pedal sits 7" above the floor ( the rod to the pedal pad was shortened many years ago.) there is 5" of travel before the brakes are fully engaged.

the system works O.K, but I don't like the long pedal travel.

I've gotten lots of ideas from members here which are appreciated. If any one of you who have added a power booster with drum brakes have any ideas I would appreciate any help. I just want to drive the truck instead of it sitting in the garage collecting dust & junk in the bed.
Thanks

[mr48chev]

Just thinking on it and are all of the shoes on all 4 wheels adjusted correctly ?

In my first year of high school auto shop in 1962 I was taught to tighten the adjusters to the point where you couldn't turn the wheel by hand and then back them off 11 clicks. That's 11 adjuster teeth. You will have some drag but that is required for the brakes to work right. I think where some guys run into problems is that they want to back the adjusters off until they have no drag. If they are backed off too far you are using all of the fluid in the bore of the master cylinder to move the shoes out to the drums before the shoes start to work.

[1project2many]

Ok, I think this problem will require a little deeper look if possible.

Mustang booster: Is this a vacuum or hydraulic booster?

Master cylinder: Did you ever figure out the bore? Are you using a drum/drum cylinder or a disc/drum version? Do you happen to have a part number?

Booster/Master Cylinder combination: CPP sells a complete kit for your truck but you said you have a Mustang booster/Master. Are you saying you bought a kit for a Mustang, or the parts in the kit for your truck are the same as what's in a Mustang?

Test step: Can you install the master without the booster to check "basic" brakes?

Thank you.

[nvrdone]

ok, to clarify -
Mr48chevy - yes the brakes are adjusted correctly. tightened till they wont turn & backed off until there is a slight drag on all wheels. 12 clicks at all wheels.
brakes adjustment has not changed from before booster / mc combo was added.

1project2many - the booster is vacuum. M/c bore is 1 1/8". its a disc / drum mc with a 10# check valve in the front line. CPP's invoice for the kit ( for my truck ) says its a mustang mc / booster. pn on the mc is29833 which crosses to a Napa # M2315 for an '82 mustang. I cant remove the booster because the bolt mounting pattern is different between the m/c & the booster.

the thing that is driving me nuts is that the brakes worked better before the new m/c & booster was installed.

here's an additional thought. after reading jweb's link to the brake article im wondering if the m/c has a check valve in the port for the rear brakes? how can I check that or should I just add a 10# valve to the rear & she what happens

thanks for all the help.