Re: Proportioning Valve Keeps Tripping?
 

Bleeders are on top, and the shuttle valve is moving to close the rear circuit. I am not sure if the prop valve switch is recentering the shuttle or if it is actually returning to center on its own.

I am curious - when the cap is off the master cylinder, and I press the brake pedal, there is a bit of brake fluid that 'shoots' up, to the point I just put a rag over it so it doesnt get all over the place. Maybe a few inches, is this an indication of anything?

Re: Proportioning Valve Keeps Tripping?
 

That's normal. Kinda like a miniature Old Faithful!

Re: Proportioning Valve Keeps Tripping?
 

If you have an ohmmeter, remove the brake warning switch, then set it on a workbench and check to see how sensitive it is. As I mentioned in a post above, the brand new one I bought would close (zero resistance) when the pin barely moved -- around .050". It could be that yours has a similar problem, so even when the pressure differential valve moves just a tiny bit, the pin moves up and closes the switch.

Re: Proportioning Valve Keeps Tripping?
 

I will check that out - Im not sure why it didnt register to me when you first said it but I will try that. Maybe as simple as a switch. Since I am away from the truck and havent tried the pedal with it running - should be pedal be harder/more firm with the booster set up when the motor is running? Or softer? Like I said I havent run it, and last I drove this truck it was manual brakes.

Re: Proportioning Valve Keeps Tripping?
 

>>should be pedal be harder/more firm with the booster set up when the motor is running?<<

Pump the pedal a couple of times and HOLD. You should have a very high, firm pedal.

Start the engine. As vacuum builds, your foot and pedal will drop.

The booster is designed to hold vacuum for 3 applications of the pedal. If the engine dies while driving, the booster should hold plenty of vacuum to give vacuum assist in order for you to get to the side of the road.

Turn the engine off. Apply the brakes 3 times. The pedal should be higher and firmer after each successive application.

Re: Proportioning Valve Keeps Tripping?
 

Well, heres an update again.

Pedal gets firm about 1/4 down, and feels OK when engine is OFF.

But, when its running, its like its a 100% dry system. Put in gear (on jack stands) and brakes have zero effect in the rear, fronts I havent checked since I didnt have it actually move. I am at a huge loss right now because I have literally bled gallons through this system. Now I am thinking the MC is bad.

Thoughts?

Re: Proportioning Valve Keeps Tripping?
 

post #34

>>My only conclusion from all of the above postings is that the Primary Section of the MC has failed.<<

Re: Proportioning Valve Keeps Tripping?
 

Do you have the tools, brake line, and fittings to bypass the combo valve? It may possible that the proportioning valve is limiting pressure to the rear WAY before it's supposed to.

Then again it might be a bad M/C like Richard says. Bypassing the combo valve will tell you where the problem is, or isn't.

Re: Proportioning Valve Keeps Tripping?
 

On old earth moving equipment, they'd sometimes use an oil can clamped to a hose to bleed hydraulic systems from the lowest point back to the reservoir.
You might try this with a big syringe from the farthest wheel cylinder. Looking for flow, and maybe trapped air. This way is messy, so rig up a drip pan and wash off any spilled fluid in the engine compartment

Otherwise, I agree with the above and you'll have to get clever with swapping parts or plumbing to isolate the problem

Re: Proportioning Valve Keeps Tripping?
 

I am in this camp as well.
So I dont have the tools handy - nor do I trust my flaring abilities at this time, so I swapped prop valves. No more brake light at all, but pedal only stops the rear tires at the bottom of the pedal (on jacks, no load)
I have tried this as well, at this point I am convinced I have no more air (I ran two gallons through the system, last 1.5 gallon had no air)

Re: Proportioning Valve Keeps Tripping?
 

UPDATE 4/3/24

I ordered a new proportioning valve to swap out in case this ends up being an issue - it has solved the brake light issue, but the brakes are only at the bottom of the pedal.

I have ensured a 1/16 clearance to the MC from the booster, and that is proper.

I am in the camp that the MC is bad. That will probably be my next item to replace.

Re: Proportioning Valve Keeps Tripping?
 

Might be the only thing wrong with the master is it’s bore is too big.
Measure it first.

And did you change your pedal ratio?

Re: Proportioning Valve Keeps Tripping?
 

The bore is 1", I would imagine that isnt a problem given this is a 'kit' set up to work with each other. The pedal ratio was slightly modified, but pushing the pedal in and out, I dont see any issue with the travel.

Re: Proportioning Valve Keeps Tripping?
 

Well, I ordered a new master cylinder last night. After that, Im all out of ideas. We will see!

Re: Proportioning Valve Keeps Tripping?
 

Some service manuals said to lightly tap the caliper with a hammer to dislodge any small air bubbles that may be clinging to the caliper wall. This could be your pressure differential problem. This was in the textbook that I used when I taught at a trade school.

Re: Proportioning Valve Keeps Tripping?
 

If there's air it's in the rear, and those are drums IIUC.
Could still be air in those wheel cylinders somehow, but I don't know if it can hide the way it might in a caliper.

Re: Proportioning Valve Keeps Tripping?
 

I would consider air in the rear or front but given Ive done so much bleeding (literally gallons), if theres trapped air it at this point is a permanent resident :lol:

So my train of thought is this:
My proportioning valve is no longer biasing to the front or rear, which indicates a internal fault in the prop valve.
Given the red brake light no longer persists, but the pedal is far weaker than it should be, I would imagine an issue before the proportioning valve exists, which would indicate the master cylinder or booster has failed.
Hopefully I will be putting this theory to the test here this upcoming week.

Re: Proportioning Valve Keeps Tripping?
 

Well, I am at a loss.

I have replaced the master cylinder, and found a broken flare on the master cylinder side for the primaries. New master cylinder, fronts bled great, rears I could hardly get any flow. Not sure why. Verified strong flow at line out of master, prop valve and out of prop valve. At the wheel cylinders, its a slow drip.

Nevertheless, I bled as best I could, still no pedal. I have now replaced all beside the booster, and I dont honestly think the booster is bad because I feel the pedal would be hard, rather than just pure air.

I am pretty seriously beat down now. I may just walk away for a bit after all this effort and time and money to be in the same place.

Re: Proportioning Valve Keeps Tripping?
 

Dent or obstruction in the hard line from combo valve to rear hose? Or maybe the hose itself or the tee? Sounds crazy.

I know someone said not to blow compressed air through the lines, but that's what I'd do at this point.

Re: Proportioning Valve Keeps Tripping?
 

>>found a broken flare on the master cylinder side for the primaries.<<

"primaries" ? Explain. The rear piston in the MC is the Primary (singular). The line flair leaked and you replaced the line ?

>>Verified strong flow at line out of master, prop valve and out of prop valve. <<

If you cracked the fittings, pumped the pedal and got fluid out, when you released the pedal it would pull air back into the lines, combo valve and MC. Explain your actual process.

Did you recheck the booster pin-to- MC depth with the new MC ?

What bleeding method are you now using ?

If manual bleeding, I suggest getting a helper and open/close those speed bleeders as though they were regular bleeders. I'm not totally convinced your speed bleeders are acting as they should.

Re: Proportioning Valve Keeps Tripping?
 

By primaries I was meaning just the master itself. So the front bowl flare on the master casting was busted. I replaced the master and the line to the prop valve to be safe.
I am using a pressure bleeder set to 15-20PSI, so no pedal actuation.
I have checked clearance with a micrometer end and a ruler, and am around the spec.
I checked the lines, no breaks or kinks, I cracked all fittings with the pressure bleeder on it, and have good flow all the way back to the wheel cylinders.

When I crack the rear bleeders, I get a rush of fluid, then it slows and eventually just completely stops. The fronts will rush out and fill a mason jar quick, rears wont even get above 1/4inch of fluid in the mason jar before it stops.

Compressed air will probably be my next step. I just cant believe how many issues I am getting right now. Its insane.

Re: Proportioning Valve Keeps Tripping?
 

I once had a pressure bleeder kit and never could make it work right. But it was great for making a mess!

How about old fashioned manual bleeding? That's where a helper does "pump, pump, pump, hold" and then you open the bleeder screw. Then you close the screw and repeat the process until there are no more bubbles. If you get very little fluid out the wheel cylinders using that method, then I give up.

I've also used a MityVac, which works fine, but as mentioned a couple times in this thread, it will suck in air past the bleeder screw nipple. Makes it look like there's air in the lines when there's not -- or probably not.

Re: Proportioning Valve Keeps Tripping?
 

Interesting behavior. Brings me back to wondering if the master cylinder has front reservoir for for rear drum brakes and rear reservoir for front disc brakes the way they are typically set up for C20.

As mentioned in this other recent thread by geezer#99 and earlier in this thread by kwmech the master cylinder may have residual pressure valves that are specific for drum or disc, and the residual pressure for drum is greater.

I don't know the confidence level that should be trusted in what the "people who sell these" saying about the design of it. I would want to see the actual specs and would like to have the tools and equipment to do measurements (like for the residual pressure, as if I would know how to measure that). It's hard to put a brake system together and trust that the specs are right. This thing you're going through is mysterious and I wish I had advice to remove the mystery and just determine the facts.

The action I would take is highly biased so you may not find it useful, biased by my preference for manual brakes on a C10. So I'd be putting a manual master cylinder that I know works for disc/drum on the firewall and call it a day. The manual master cylinder has a deeper bore for the push rod. Kit type master cylinders often have a deep bore and an adapter pin that fit in the push rod hole to make it compatible with a booster that requires a shallow push rod hole. I'm sure there's not very much to adapting a deep push rod hole to a shallow one, it's just a small rod of metal. So perhaps one of those manual master cylinders could be adapted for power, or a different power master cylinder that somebody here on the forum can attest to working on a C10 could be used. 71-72 C10 that came from the factory with disc/drum had 1.125 bore regardless of whether they were power or manual, and that has been shown to us by Keith Seymore right from the engineering specs he posted. A MC with 1 inch bore is not to spec for a C10, I'm not saying it won't work, it just reduces my confidence in the people who sell these kits and makes me wonder what the heck. Anyway I ramble, sorry too much rambling, saying bye now and I hope you find the solution.

Re: Proportioning Valve Keeps Tripping?
 

I agree with regular manual bleeding is best, it's just simple. I tend to use the one-man method that does not require a helper. It is where you put the hose at the bottom of a jar that has some brake fluid in it, and open the bleeder. Then once that little bit of air is pumped out of the hose by brake pedal action, no more air can be sucked back into the system. Pump the brakes gently and slowly until you mostly fill the jar, and replenish the MC periodically. Then close the bleeder before removing the hose. O'Reilly auto parts have one-man bleeder kits, but it is also OK to just use a coffee can and submerse the end of the hose. If you think you're getting any air past the bleeder threads, you can remove the bleeder and put a little chapstick on the threads, and re-insert the bleeder screw.

Re: Proportioning Valve Keeps Tripping?
 

About master cylinder built-in valves, I cut this from a post on The H.A.M.B.

An easy way to check any dual master is to take a paper clip or similar small diameter object and slowly push it into the output ports. If you feel a spongy resistance, you are pushing against a residual valve. If none are present, the object will easily go into the master bore.

I seriously doubt if yours has a built-in valve or valves. Everything you need is in the combination valve.

Re: Proportioning Valve Keeps Tripping?
 

That is interesting information. There's too much general information that's confusing, and not enough actual technical specifications about what is in each component. I understand the factory combination valves don't have residual valves, they just have the shuttle switch, metering valve (for 71 manual brakes they didn't have one, but for other applications they did), and proportioning valve. Who knows what is in the chinese brass block style replacement combination valves. I'm kind of doubting there are residual valves in there. The residual valves would have been in the master cylinder way back when, but there is other information on the internet about modern wheel cylinders not needing residual valves any more and who know the specs on the master cylinders they are selling in parts stores (or anywhere).

Re: Proportioning Valve Keeps Tripping?
 

Did you measure the bore yourself or just look at a spec sheet?

Re: Proportioning Valve Keeps Tripping?
 

I tried the two person method tonight, and I got little to no pressure, I tried even punching the pedal to rush fluid - nothing. I have found at the tee on the rear end, one side will not drip any fluid or maybe once when I crack the line open, one side will run like a river. I am thinking I have an issue there. Which is so strange - I had no issue before the master cylinder swap.

I am at the point I may end up buying some special brake pressure measuring tools to help determine some of my issues. At this point, I may be disassembling the rear ends brake lines and tee, to check for blockage, if none is present, my next move will be diagnosis with specific tools.
I normally do the one man method, but I cant even get a small amount of fluid from the rear. Its mind boggling. Even punching the pedal does nothing. I figured a pressure rush would at least make it do something, but nothing at all.
I will check this next as well.
These measurements are from their spec sheets. I am not versed really in most brake system specifications, more so a general sense, which seems just enough to get me into trouble at this point

Re: Proportioning Valve Keeps Tripping?
 

Update;

Still no pressure at the rear wheel cylinders, no amount of two person bleeding nor pressure bleeding really yields me any more fluid than the other.

I have check a few things;
Fittings at master for flow when the pressure bleeder is on - good
Fittings at the proportioning valve when the pressure bleeder is on - good
Fittings at first union when pressure bleeder is on - good (both sides of union)
Fitting at the hard line to soft hose when pressure bleeder is on - good
Fittings at the tee for the rear end distribution when pressure bleeder is on - passenger side - little to no flow -- driver side, good
Fitting at wheel cylinder on passenger side - no flow
I cannot check the fitting at the driver side wheel cylinder as the nut is rounded, but no flow at bleeder. Even removing the bleeder from both sides on the rear end, virtually no fluid comes out.

My only conclusion is I have some incredible blockage here, maybe the soft line failing from being unused for so many years? I really dont know.
I may just buy a new soft line & tee, with new rear hard lines just in case for this weekend when I continue deeper.

Though this is super discouraging, I really want this truck on the road.
and on we go

Re: Proportioning Valve Keeps Tripping?
 

I offered some advice early on and saw that you were getting better advice from others, but I've been following. I had suspected your rear rubber hose, and I couldn't find in your posts if you had replaced it. I'm glad you were able to troubleshoot your brake lines, and thank you for posting on your updates. You're helping some future 67-72 GM truck enthusiast that may have similar issues. The tee can be cleaned unless it's part of the rubber hose. I hope your brake kickass after all that you've had to do.

Re: Proportioning Valve Keeps Tripping?
 

Thats exactly why Ive been updating with what Ive tried, to (hopefully) when this is fixed (Lord I hope soon) help guide others who may encounter any level of the issue I am running into. I know I use this forum daily for all kinds of stuff. Its an amazing plethora of information. I also own a 54 Bel Air and info on that thing is scarce and spotty compared to these trucks.

I did replace that proportioning valve, and I do believe the old one was faulty and too sensitive, as this new one is more firm in its shuttle bore, the old one could be moved by a paperclip. So that was correct in suspecting there was a fault there.

Re: Proportioning Valve Keeps Tripping?
 

Well, that's not good. Not anything to fail there, so maybe a blockage of some sort? When I mentioned checking it, I was grasping at straws. Remove it, soak it, and blow it out.

Re: Proportioning Valve Keeps Tripping?
 

I plan to do just that. I find it really weird, especially with 25PSI pushing on it, but we will see!

Re: Proportioning Valve Keeps Tripping?
 

I think your problem is related to m/c bore and pedal ratio.
Here’s some info for you.


https://www.speedwaymotors.com/the-t...e-system/30895


https://www.onallcylinders.com/2014/...der-bore-size/

Re: Proportioning Valve Keeps Tripping?
 

I am working towards that now. I will consult those references and some stuff on Wilwoods site and see what I can deduce.

Re: Proportioning Valve Keeps Tripping?
 

UPDATE 4/18

Since my last update, I have bench bled the master (again), replaced the rear brake hard lines and the single soft line, and cleared the tee of debris.
The rear lines had some junk, nothing significant, and the tee had some as well. The rear hose flowed fluid about as much as I would say the new one does. Was this a problem? Maybe, but either case, the hose was 5 years or more old, and the lines were beginning to strip on the line nuts. So not regretting I did those in any capacity.

These made no impact on the outcome.

I still have zero pedal.

I checked the clearance for the push rod from the booster to the MC, and it moves immediately when pedal input is applied, and moves smoothly.

For S&G I adjusted the pushrod WAY out to preload the brakes, see if I could even stop the trucks rear end while in the air, it took 5+ pedal pumps to stop it, but once stopped it did not allow the wheels to free spin in gear again, as if they were locked on. I did not apply throttle to see if it would do anything.

This is honestly the most response Ive gotten from these brakes, and I still have no idea of the operation of the front brakes as I havent had enough confidence to even allow the truck to move under power.

So, next I will install block off fittings on the master to see if I can have any pedal when flow is 100% sealed.

If this master is bad, I may have to move to some sort of Wilwood or something. I would prefer not to, as it will not fit the trucks looks whatsoever, but it may be my only option at this point.

Re: Proportioning Valve Keeps Tripping?
 

Change your pedal ratio.
Check where your connection pushrod is on the brake pedal and Drill a hole 1 1/2 inches lower. Attack the pushrod there and test. Your pedal ratio should be close to 4 to 1 then.

Re: Proportioning Valve Keeps Tripping?
 

Can you post pic's of your brake calipers as you have them installed? I'm just shooting in the dark here....on a thread a while back there was a situation as yours where the owner kept stating the bleeders were on top, and they where.. But they were not on top of the piston bore to remove all the air. He finally rotated or swapped the brackets which put the bleeder actually on top and problem solved.

Re: Proportioning Valve Keeps Tripping?
 

Yes, that should eliminate the master cylinder.

Also, I think the problem is something other than pedal ratio or master cyl bore size. As an example, when I installed 85 C10 spindles and disc brakes on my 69, I actually drove the truck for at least several weeks with its stock manual drum/drum master cylinder, and the brakes worked pretty well. Sometimes I wonder why I went ahead and installed the 71-72 master cylinder, booster, brackets, and pedal linkage! However, the system has worked great since then, minus some issues with the 50+ year old combo valve, and brain fade on my part when bleeding brakes recently.:o

Re: Proportioning Valve Keeps Tripping?
 

>>I checked the clearance for the push rod from the booster to the MC, and it moves immediately when pedal input is applied, and moves smoothly.<<

That doesn't tell you anything about the correct booster pin length.


>>For S&G I adjusted the pushrod WAY out to preload the brakes, see if I could even stop the trucks rear end while in the air, it took 5+ pedal pumps to stop it, but once stopped it did not allow the wheels to free spin in gear again, as if they were locked on. <<

Adjusting the pedal rod longer just makes a problem with incorrect booster rod worse.

MC pistons must be able to return to seated position so that ports are opened to allow fluid to return to reservoir when pedal is released.

Adjust pedal rod for freeplay.
Adjust booster rod with actual measurements. If you don't have the measuring tool you must be able to move the MC far enough to take measurements.
A - B = C, which is what the tool does.
After "C" is determined, reduce pin height ( C ) by 1/32 to 1/16".

I set the clearance at 1/32 on a new booster for my Vette. After a couple months, I noticed at stop lights with slight grade, the vehicle would no longer roll with brake off. (manual trans) The seats inside the booster had settled with age and spring pressure. The pin had extended past my original setting and was holding the MC slightly engaged. I won't set one that close again.