Technical Need some help with brake issue

Hello,

My car is finally done, I've taken it on a couple test runs, have had a few issues and fixed them, but I have a brake issue that's perplexing me...

I'm using a GM proportioning valve (see pic) and brand new disks on front, drums on back. Power assist booster under the floors.

The brakes feel great while driving, but after 15-30 mins, the front pads will lock to the disk and not let go. The 1st time, I cracked on bleeder and it was fine, made it home no issues. Next time I went out, drove 30+ mins fine, got 2 mins from home and full brakes on the front again. Released the bleeder and fine again...

3rd time, drove about 15 mins, had no issues, parked in the garage and after 10 mins, tried to turn the wheels, locked again. After a few hours, and not cracking the bleeder, they started to turn, but still a fair bunch of friction. Front discs are very HOT.

I'm wondering how much of this is a heat issue, it's been very hot here... Not sure what I can do to remove a bit of brake pad material to avoid heat expansion?

Thanks for any advice

 

Check that you have brake push rod clearance; i.e. MC piston is fully released [out] against the retaining ring otherwise the piston will block ports and when the brake juice heats up and expands it can't return to reservoir.

 

Return spring on the brake pedal?

 

The rod from pedal to booster, as well as the rod from booster to master. The booster/master rod usually has a very fine tune length to it, and you probably need to know the specs to adjust it.

 

99% sure your setup is not fully releasing the master cylinder. It is probably a push rod adjustment as others have said.

 

im calling defective proportioning valve...made in you know where im thinking...

 

Do you have a brake line, brake cylinder or proportion valve stationed close to an exhaust pipe or muffler??

 

That is a combination valve I believe?

 

Thanks so far, awesome info. I have checked that the pedal is returning fully, it seems to be. I will check for any adjustments, that's a good idea.

I checked the car today, it's a cooler day and 2 full days since I drove it, tires turn just fine. We haven't done anything at all to it, haven't touched the brake pedal, or bled any fluid...

If the pads are too tight (they are brand new) can some material be taken off to allow more room for heat expansion?

 

If it is not the push rod like others are suggesting it may be what is stated concerning the mounting of the caliper to its mount. Prior to pads being placed into the caliper, the disc needs to be centered in the caliper when it attached to the mounting bracket.

 

We know nothing about what type front brakes it has...are they GM type calipers that mount on bolts, and slide side to side as needed? or are they Wilwood 4 piston calipers that need to be shimmed to get them centered on the rotors? Or ????

After he checks for pushrod free play, maybe we can get into the calipers...

 

Had the same problem on my '63 chevy pickup, it had been "late-modeled" before I got it with the front suspension/rear end/brake master from a '73 or so chevy pickup. Once the engine got up to temperature the front disk brakes would lock up, I could get the pickup rolling again by cracking open the brake bleeders on the front calipers to release built-up fluid pressure but by the time I'd driven a block or so the problem returned. The fix was to move the combination valve from where it was right next to the driver's side exhaust manifold to the front cross-member underneath the radiator where it could get some cooling air.

 

Yes...

 

GM style

 

I see your in Vancouver. That could explain things. It has been hot rather then gray and rainy.
But seriously, Post 2, 4 and 5.

LMAO...returning fully and seems to be in the same sentence doesn't build confidence.
Loosen the two master cylinder bolts. If it falls away rather then pushes away, the rod is too long. That's the case, #2,4,5 and me are wrong.

You shouldn't have to, you did that to get it home didn't you? The again, maybe it's that proportioning valve getting to hot? Well, that you have to move as in relocate or build a shield.
But as Junkman pointed out as he's experienced it, your picture to me says it's far enough away?

 

Found a pic of the push rod that came with the brake cylinder kit. I'm not sure how anything in the rod can be adjusted, but I'm open to ideas. I went under the car (2 days ago) and had someone press and release the brake a bunch, it seemed straight, no binding, and seemed to come back out...

 

The piston in the master cylinder needs to return to the fully "out" position when you release the brake pedal. If it stays "in" a little bit, it will cover the compensation port and not allow the fluid to return to the master cylinder, causing the brakes to remain engaged.

That's what you need to check. Make sure there's some free play. Perhaps measure how far the pushrod sticks out of the booster, vs how deep the hole in the master cylinder piston is, all measured relative to the same surface.

 

Disc brake pads should not be ground or sanded for a problem like you described, the problem is fluid expansion due to heat, improper master rod(s) adjustment, the pedal should have free play, hot or cold , about .250" to .500", the caliper piston will "relax" as soon as pressure is released, providing bracket is installed correctly.

 

Those combination/'proportioning' valves are a waste of time and money. Non-existent before '67, these came in with the now-mandated split braking systems and IMO were primarily a place to install the also-mandated brake warning light switch. As I have yet to see anyone wire in that light in a custom install, that leaves the metering/proportioning circuits as the only 'useful' parts. But are they useful? The OEMs had/have literally dozens of different part numbers for these valves, representing all the myriad combinations of brakes/weight distribution/wheelbases with individual 'tuning' for the various combinations. What's the aftermarket sell? Three 'generic tune' versions (drum/drum, disc/drum, and disc/disc) with no pretense of being 'right' for your combination. Just try to get a coherent response as to why these are 'needed' (other than to enrich the parts suppliers).

Jaguar didn't use these at all up until 1990 (when they switched to ABS), the calipers were plumbed directly to the master cylinder. They used a different solution for the warning light.

Jaguars weren't known for poor brakes....

 

What you are holding in the photo is not a BOOSTER PUSH-ROD but a MC PISTON ADAPTER to use a deep bore MC on a shallow bore MC piston design.

Your problem(s) are most likely in the rods adj (MC and BOOSTER). Was this a complete kit that came assembled or did you assemble from service parts? If a kit, the booster push-rod should have been adjusted correctly by the vendor.

You cannot shim or remove lining material on a sliding/floating caliper assembly as once you energize the calipers, they will over-ride any attempt as such.

Once you have straightened this out, flush and bleed the system as the fluid in the caliper(s) is overheating and causing the drag (due to the incorrect MC/BOOSTER ADJ), and is most likely burnt. The pads/rotor overheat due to drag and the fluid expands causing the lockups. You don't want it finding it's way upstream (as in replacing the pads by compressing the caliper pistons and not opening the bleeders to expel the burnt fluid).

If you need more info on how to adjust he push-rods, just ask. Is the MC (GM) a CORVETTE DISC/DISC style? You have DISC/DRUM, correct?

EDITED - To Hopefully Make It More Concise

 

Before the 1967 mandate, the valve(s) were separate. The PROPORTIONING VALVE (which is necessary) and the METERING VALVE were on different circuits. The PDV (PRESSURE DIFFERENTIAL VALVE) (along with DUAL RESERVOIR MC) was added by mandate in 1967 to provide an impending failure warning lamp.

FORD didn't go to an actual COMBINATION VALVE (containing two or more valves) until the 1971/72 model run. Not sure about GM, MOPAR or AMC).

 

My 41 Cadillac with stock drum brakes did the same ‘lock up’ after I rebuilt the stock master cylinder with a new kit. i shortened the push rod between the pedal and the master cylinder. gotta have a little slop. works good now.

 

No. Disks self adjust. Anything removed will be compensated for on the first push of the pedal.

Pushrod keeping M/C from fully returning is most likely.

 

Heat shield around that proportioning valve may be needed.

 

That’s not the pushrod. That’s the insert that goes in the M/C plunger to work with a short pushrod, generally used with a vacuum booster. The same M/C can be mounted without the booster, remove the insert, and use a long pushrod. These are aftermarket “muscle car” parts. The two pushrods are non adjustable, so *should* be right assuming all parts from the same supplier and matched.

When assembled, there should be slight clearance between the end of the pushrod (sticking out of the booster) and the cupped end of that insert. The clearance allows the M/C plunger to fully return when the pedal is up.

If the pedal doesn’t fully retract, it could also keep the plunger from fully releasing. That’s why people are asking about the return spring.

 

That is more of a safety valve it will turn on a warning light and block off fluid from going to the failed half of the brake system a proportioning valve will change pressure values to one end. I would look at the push rod or possibly the booster. The next time happens try removing the lid on the reservoir and tap on the side of the master cylinder and look for a rush of fluid back the reservoir.

 

If cracking a bleeder releases the brakes, the problem is in the hydraulic system. Your pedal setup is not allowing the mastery cylinder to fully release. You can't just play with the pedal and say that it is returning okay. You need to actually take things apart and verify that there is no preload on the master cylinder piston with the pedal in the released position.

 

All the PDV (PRESSURE DIFFERENTIAL VALVE) does is to allow a pintle to move towards a failing/failed brake circuit to energize a warning lamp. It does not cut off fluid pressure to the affected circuit.

EDIT (30SEP2022) -

It will somewhat limit pressure but will not fully stop it. One that is tripped will drive you crazy while trying to bleed the brakes until you realize the pintle needs to be centered.

The safety factor is in the dual reservoir MC design, not the PDV. If there is a catastrophic hydraulic failure on one circuit, the operational circuit (if one) should allow a safe stop to the shoulder of the road.