Tricky Brake issue, help needed brake gurus

Background: Working on a '56 GMC pickup for a customer. Disk/disk setup with 12" GM metric in front and stock Metric disks in rear. Aluminum master & booster under the floor. Using Wilwood prop valve & 2lb. disk check valves.

Problem: After driving the pickup for an extended amount of time the brakes begin to drag. After time the pickup will totally stop itself when you push in the clutch. The brake lights will come on & stay on. Some how it's building pressure in the system causing the brakes to apply on there own.

Already investigated things: We've check pedal free play, made sure the e-brake was properly adjusted, checked all soft hoses for restrictions. Made sure fluid was going out & back into the mast cylinder. Checked for areas where exhaust might be to close to the lines.

I'm asking for some serious help as I've exhausted most of my ideas and aren't getting far (literally). I'm hoping some fellow savvy HAMBERS will have some new ideas to try/check. Thanks! I might post pictures of the system if needed.

 

Actual clearance is a little more involved to check when using a booster; but if the master cylinder piston returns against the snap ring and you really have clearance between the pushrod and piston with a return spring to maintain it; the only thing left is the master cylinder itself.

99.99% of the time I came across this problem, it was lack of clearance caused by mis-adjustment or pedal linkage hang-up.

 

I know you said that the fluid is returning to the master cylinder, but the pistons in the calipers can stick. Maybe just the fronts or backs.

 

Is the exhaust close to the master cylinder and/or tubing? Maybe the exhaust heat is making the fluid expand and thus putting your brakes on without the pedal being depressed. Do the brakes work ok when everything is cooled down?

 

did you try cracking a bleeder to isolate which axle/side?

 

I've run into this having too much brake fluid in the master cyl 3 times. If it's filled right to the top or damn close to it it *may* not have enough room to expand when/if it gets hot causing the brakes to apply themselves.
Sounds like BS, but it happened to my old 53, a Vette i had and a guy's Harley that came into the shop.
Pulled a little fluid out leaving about 1/4-3/8" or so for expansion and it solved the problem.

I'm ***uming the truck has a hyd brake light switch?
Just an out there thought.

Tony

 

Possibly, faulty check valves.

 

Could there be a problem with the booster??
Ive seen them actuate themselves because of faulty internals.

 

My first thought.

 

Im almost positive it's not the exhaust. I even wrapped the only spot it was sorta close & that did nouthing. I wish it was that though.

 

Is the proportioning valve ok? Is it valving correctly? If you can byp*** it easily it is worth checking.

 

The last time I had this problem It was the compensating port stop up int the master cylinder. Replace master and fixed the problem. make sure the exhaust is not heating up the master cylinder.

 

Remove the 2# check valve and see what it does.

 

Maybe clarify my earlier post.

A. If the brake lights are on, there is pressure in the system.

B. If there is pressure in the system, then the master cylinder is
not releasing the fluid.

C. Diagnose cause.
1. Inadequate clearance between the pushrod and booster.
2. Pedal not returning, hang up or lack of return spring.
3. Improper clearance or mis-match between booster and master cylinder.*
4. Defective master cylinder.
5. Defective booster (may be possible, never seen it tho).

* May be possible to to space the master cylinder away from the booster
with washers as a test with having to take everything apart.

 

What he said . . . .

 

How close to the exhaust are the hard lines run?

 

You should be able to easily p*** your hand between the exhaust and brake component.

If a brake line gets real hot, it gets porous and you actually see the fluid boil through it. I had that happen on a Chevy truck where the main line from the master was too close to a header tube.

So for a brake system to be stuck on, I doubt its a heat issue. I'd think that there is enough tubing to dissipate a local overheating condition. Unless the master itself is getting cooked.

I would still go around and crack bleeders to see what axle is locked up.

Bob

 

If you have rubber brakes lines they will do that,never pinch them.

 

Are you sure the master cylinder is set up for disc brakes. Check to see it is has a residual valve.

http://www.thebrakeman.com/valvetechi

 

went thru this some years ago on an o/t dodge van, booster was replaced, and we ended up with the same issue you describe. we were missing a spacer, although i can't remember if it was between the booster and mounting bracket- booster was sideways with a bell crank set up-or between the booster and M/C, but esentially the booster was keeping pressure on the m/c causing the brakes to act like you describe.

it seems if pedal free play is good, then you'd have to look at m/c to booster issues. i've had bad boosters cause the brakes to apply due to ruptured diaphrams. one would think if it were an actual booster diaphram/ vac. issue, that would go away with removing vac to booster. if they still drag/lock, it may be the pushrod inside the booster is too long for the m/c, causing the piston in the m/c to never fully return.

hope this helps out.

 

I had a similar problem a couple months ago. I started a thread about it. I use drum drum and non power but anyway the master that is under the floor didn't have the second hole drilled in the reservoir to allow the fluid to return properly into the reservoir when the pedal is released. Basically it acted like a check valve itself.

Replaced the master I got from speedway with a dorman and problem was solved.

 

Check to make sure that the piston in the master cylinder fully retracts so that the ports in the reservoir are open and not covered by the piston.

(o[]o)

 

You need to diagnose where the residual pressure is building up. Start cracking open bleeders and line fittings to figure it out. I would start at the master cylinder.

 

I've isolated the problem to start at the front brakes together. The rears also ingage after. The front lines & calipers are warm to the touch. The aluminum master is also warm. You can crack both front bleeders and let off pressure. The exhaust in the front is 6"-8" minimal from the lines in the tightest area. I'll post pictures later. The master plunger stops on the snap ring with extra free play. The pedal ***embly is a no limit engineering piece.

 

So the master returns to the snap ring while attached to the booster? Or while off the booster? Just wondering, as we all know, the harder the problem is, the simpler the solution turns out to be.

 

Is there an extra residual valve built into the master cylinder itself?
Is the pedal returning to to full up position or does it need a return spring?

 

haha, yea that's usually the case.

 

I put a return spring on it day one.

 

here's the pictures.

 

Have you double checked to be sure the front residual is a 2 pound valve and not 10?