Proportioning Valve Keeps Tripping?

[RichardJ]

He stated that he is using the speed bleeders. They come with a thread coating, although this coating could have deteriorated. A common problem with vacuum bleeding is sucking air past the bleeder threads unless the threads are wrapped.
He also said he has tried a pressure bleater.

I could see a problem with manual bleeding, if debris is holding the Speed Bleeder Valve open and the valve is not closed, each time the pedal is withdrawn. This wouldn't affect vacuum or pressure bleeding.

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

[Rust_never_sleeps]

You could always let it gravity bleed :shrug:

[KyleSeal]

Originally Posted by 54blackhornet

Kyle I applaud your persistence ! Keep at it ! Lots of people will learn from your experience..Jack

Thank you. I plan to get this fixed and report back my findings! Fingers crossed.

Originally Posted by MikeB

If air is somehow creeping into the system (which I doubt), that means you would have a fluid leak. Have you seen signs of that anywhere?

FYI, if you are pulling fluid trough the bleeder valves using a vacuum pump like a MityVac, you will almost always see bubbles from air creeping in between the hose and bleeder valve.

I verified at each fitting that there is no leak, but I have seen a lot of documentation for the bores of the wheel cylinders to pull air and foul that section of line with air. I believe it was Stocker on this board who had a similar issue, and he took it to a shop and they replaced the wheel cylinders in the rear and it was solved. Plus I am thinking starting with wheel cylinders will be the most inexpensive option at this point - next would be MC.

Originally Posted by dmjlambert

What are the full specs for the kit, or webpage where it is sold?

This is a CPP front brake disc conversion kit, Inline Tube prebent lines, stock rear drums and a kit from Tom's Classic's for the booster/MC/PV

Originally Posted by 72SB

It is not uncommon to pull air from bleeder nipples being loosened to bleed at the threads. A thin coat of silicone on the bleeder nipple threads prevents this. Do not use Teflon tape.

Ive tried the silicon before with not a ton of luck, but I use a small run of teflon tape on the threads (not the tapered seal and not on line fittings) as some of the folks at Hagerty suggested. Why I did this, is because when I loosen the bleeder I can wiggle it some and Im certain air is coming through when bleeding. With a small layer of teflon, no deflection is noted.

Originally Posted by RichardJ

He stated that he is using the speed bleeders. They come with a thread coating, although this coating could have deteriorated. A common problem with vacuum bleeding is sucking air past the bleeder threads unless the threads are wrapped.
He also said he has tried a pressure bleater.

I could see a problem with manual bleeding, if debris is holding the Speed Bleeder Valve open and the valve is not closed, each time the pedal is withdrawn. This wouldn't affect vacuum or pressure bleeding.

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

I have limited knowledge of MC operation in all honesty - would the primary section effect the rear brakes? Rear bowl is rear brakes, front bowl is front brakes. My front brakes are perfect and have zero bubbles.

Originally Posted by Rust_never_sleeps

You could always let it gravity bleed :shrug:

I have tried this as well. Still no dice unfortunately.

[MikeB]

Might be a PITA to bypass the combination valve with temporary hard lines, but that would probably tell you where the problem is. Of course you would need the proper bending and flaring tools, along with fittings.

Is still don't see the problem being a wheel cylinder or something else "sucking in" air.

[airdale94]

Have you tried adjusting the push rod between the the boostewr and master cylinder

[KyleSeal]

UPDATE 3/28/24

I replaced the wheel cylinders for the rear axle, and the fluid is coming out completely air free now. I ran a ton of fluid to be sure with a pressure bleeder for extra insurance.
The pedal feels more solid at this time.
NOW, I am having a brake light come on the dash past 3/4 pedal travel. BUT, when this happens, when I bring the pedal back up past that area, it clears itself. So I am thinking I have or may be dealing with separate issues. Possibly the MC bypassing itself? I at this point get sick at the idea of bleeding brakes. LOL.

Any further pointers or ideas?

[MikeB]

Originally Posted by KyleSeal

UPDATE 3/28/24

I replaced the wheel cylinders for the rear axle, and the fluid is coming out completely air free now.

But the originals were not leaking? I don't know how they could have held or drawn in air. (Play Outer Limits theme song here.)

That shuttle valve is acting very strange. Could it be moving to the front? Are the front calipers installed with the bleeder screws on the top?

[KyleSeal]

Originally Posted by MikeB

But the originals were not leaking? I don't know how they could have held or drawn in air. (Play Outer Limits theme song here.)

That shuttle valve is acting very strange. Could it be moving to the front? Are the front calipers installed with the bleeder screws on the top?

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?

[MikeB]

Originally Posted by KyleSeal

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?

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

[MikeB]

Originally Posted by KyleSeal

UPDATE 3/28/24
NOW, I am having a brake light come on the dash past 3/4 pedal travel. BUT, when this happens, when I bring the pedal back up past that area, it clears itself.

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.

[KyleSeal]

Originally Posted by MikeB

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.

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.

[RichardJ]

>>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.

[KyleSeal]

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?

[RichardJ]

post #34

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

[MikeB]

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.

[Rust_never_sleeps]

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

[KyleSeal]

Originally Posted by RichardJ

post #34

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

I am in this camp as well.

Originally Posted by MikeB

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.

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)

Originally Posted by Rust_never_sleeps

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

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)

[KyleSeal]

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.

[geezer#99]

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?

[KyleSeal]

Originally Posted by geezer#99

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?

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.

[geezer#99]

Originally Posted by KyleSeal

The bore is 1", I would imagine that isnt a problem given this is a 'kit' set up to work with each other.

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

[KyleSeal]

Originally Posted by MikeB

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.

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.

Originally Posted by dmjlambert

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.

Originally Posted by dmjlambert

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.

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.

Originally Posted by MikeB

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.

I will check this next as well.

Originally Posted by dmjlambert

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).

Originally Posted by geezer#99

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

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

[KyleSeal]

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

[425HP409]

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.

[Rust_never_sleeps]

Originally Posted by 425HP409

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.

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.