128racecar

Last summer I redid the brakes on my 72 350 auto coupe. Everything with new stock parts- master cylinder, all rubber lines, rotors, calipers rebuilt by Lone Star with original type seals, E-brake components at rear and new cable. Bled with correct fluid per forum recs. Worked great last summer. Had proper feel at pedal with no pulling and dragging. Got the car out today for a drive. Brakes felt normal first 10-15 blocks. then I noticed it took more and more gas to get rolling after a stop. When I got back home, I jacked up all four wheels. Dragged equally on all four wheels, taking a fair amount of pushing on each tire to get it rolling. E-brake seemed to set and release correctly. Brake fluid level unchanged before and after drive. Just rechecked car after sitting/cooling 30 minutes- now rolls normal with no resistance.

Ideas?

Marc in Indy

v2racing

Make sure you have free travel at the master cylinder. If there is none, the piston inside can block the fluid return hole and cause pressure to build up in the system and lock up the brakes.

Mike

DUB

You never mentioned if you have manual brakes or power brakes because the brake booster can hang up and apply pressure to the master cylinder and slowly bleed off over time also.

DUB

128racecar

128racecar

Okay, brake pedal now has normal travel and no drag on the wheels, without me doing anything. Will drive several times this week to see if they drag again. If not, this thread may be of use to someone who has their brakes dragging after being up on jack stands with the wheels unsupported for long periods.

Marc in Indy

128racecar

Brakes still dragging after driving 1-2 miles. No drag when cold, but can't push the car an inch when hot and in neutral. Checked the brake pedal free travel- right at factory manual specs when cold/not running, but does seem diminished when hot/running. Checked the master cylinder to make sure no blockage of the return ports in the reservoir. Again, all calipers redone last year, along with all new hoses. I was wrong in the original post- master cylinder NOT replaced. Also has original power brake booster. If I disconnect and plug the vacuum line to the power brake booster, would that help sort out if the PB booster is dragging or not?
Again, thanks for chiming in with whatever ideas I should check out.

Marc in Indy

doorgunner

As another member stated, there should be up to 1/16" gap between the plunger that sticks out of the front of the booster where it fits into the master cylinder. Also, there needs to be a similar amount of free play where the rod from the brake pedal to the rear of the booster makes contact. I had a similar problem because I did not allow a "gap" at either location causing the pads to drag on the rotors once the brake system heated up.

Other members can correct any inaccurate info I have posted.

Peterbuilt

If all 4 wheels lock up you might want to look at the gap between the booster rod and master cylinder but you did not change those parts so skip that for now.
Are you sure all the wheels lock up or could it be only one?
If only one the new rubber line may be bad.
Open the bleeder on the locked wheel and if fluid comes out under pressure the rubber line is suspect.

128racecar

I'll double check both clearances, but since I didn't touch those when I redid the brakes, it would be odd they would now be a problem. There is definitely the correct clearance on the pedal push rod to booster.

It's definitely all four wheels locked up. I jacked up each wheel individually while hot to verify they were stuck. I'll do it again next weekend, and while I am at it, pull each wheel and crack open the bleeder to see if that frees up that wheel. That way I can verify it something central to all four wheels, ie the master cylinder or booster. It would be odd to have all four calipers freeze equally while hot, and then relax while cool, but I'll double them just the same.
Thanks!!!

Bigeddie

I had the same issue with the brakes but that was an aftermarket hydroboost/master. It ended up being the preload in from the rod that goes into the break master.That there is your culprit. If i remember the stock booster rod is adjustable but im not sure nor do i remember.

Danish Shark

Very easy to test if the push rod is the issue. Just loosen the two nuts holding the master to the booster and check if you can move the car.

Tonio

Brake fluid expands when it gets hot while driving. If it has nowhere to go, it pushes on the piston cups in the calipers. So I'm thinking that the return port is either clogging up, or the piston seal is covering it due to lack of pushrod clearance.

HeadsU.P.

Not for nothing, but does the brake pedal return to the proper height after release? If not, climb under the dash and inspect the brake pedal return spring.

bobs77vet

that's what I am thinking also.....after the car cools down the vacuum from the booster bleeds off.....so right now the booster thinks the brakes are slightly being applied

128racecar

All your replies on the forum are great! A progress report and a question:

Checked brake pedal free play again. Set at longest factory spec- 1/4" free play at pedal. Checked brake return spring. No binding of pedal or spring, full return of pedal with every push.

First test drive- got brakes dragging after 10 blocks (six four way stops). Jacked up all four wheels- almost locked solid. Released bleeder on left front wheel. Now all four wheels roll easily. So that eliminates anything to do with my 4 rebuilt calipers and four new brake hoses.

Second test drive- again, wheels locked up after 10 blocks. This time, loosened the nuts holding master cylinder to power booster 1/4". All four wheels now roll easily again.
So now what appears to left are:
1. Brake booster rod to master cylinder clearance too tight.
2. Internals of booster dragging, as DUB suggested as a possibility.

So how exactly do I measure this 1/16" clearance between the end of the booster rod and the piston of the master cylinder? Since the master cylinder fits into the booster, obviously it can't be visualized. Do I use some modeling putty on the back of piston of the master cylinder and assemble it, the disassemble and remeasure, like checking valve to piston clearance when installing a high lift cam? The factory shop manual has NO mention of this. I am open for all suggestions how to measure this correctly. If this comes out to be the correct spec and it still drags after 10 blocks, the next step is a new power brake booster, correct?

Thanks guys!!!
Marc in Indy

JayRay

Peterbuilt

Can the master cylinder to booster rod gap change by itself?
I doubt it, here's a how-to:


My guess is that the booster has gone bad.
Remove and plug the vacuum line to the booster and go for the same ride.
It will take a lot of force to the brake pedal but if the wheels no longer lock up. you found the problem.

7T1vette

You have a problem either with the master cylinder or with the brake booster. If all four wheels are getting a "false" apply signal (regardless of how strong or weak that signal may be), only the booster of master cylinder could make that happen. Not sure how to suggest you diagnose which is at fault (or why), but I'm pretty convinced that you will find the problem there.

DUB

As I wrote I have 'gut' feeling it is in the booster.

NOW..on extremely rare occasions which is why I had not mentioned it is that the plunger in the master cylinder is sticking and NOT going back all the way which a person can see when they take off the master cylinder and the plunger is not pressing against the snap ring that keeps it form coming out.

***REAL WORLD EXPERIENCE***

Regardless if it as on a 1985 or not. I had a Corvette that the brakes hung up on and when the customer drove it to my shop and pulled it up under my car port. I popped the hood and could HEAR the brake fluid boiling and in a few moments the front brake hose blew off the caliper. And when it was checked out...it was the booster that caused the problem.

DUB

128racecar

Latest progress report:
Pulled the master cylinder. Subtle leak in the back of the piston seal, resulting in some corrosion buildup that limited the piston from returning all the way back to the snap ring- probably accounting for at least 1/8". Add to that was the rod from the booster was at least 1/4" too far out, even further when you figure in the needed 1/16" free play. My guess is that last year, when I first got it driving with the new calipers, I never drove it more than 3-4 blocks on test drives. This year I was going on longer 10-15 block test drives, so long enough for the pressure to build up and lock her up.

Regarding another query above- fresh DOT 3 from day 1, never DOT 5.

Good news is, I have a fresh remanufactured AC Delco master cylinder on hand. I've already set it up for the correct PB booster rod-to-MC clearance, and will try to get it bench bled and installed in the next couple of days. At the minimum, DUB was right about corrosion blocking the full return of the MC piston. Add in where doorgunner and others were correct about the PB booster rod to MC piston clearance, and I hope that is the end of it. I'm hoping I don't end up with the trifecta of DUB's mention of a possible bad PB booster. I'll know once I get the new MC in, get her bled, and get a longer test drive in. Worst case scenario is I am laying on my back in a couple weeks, trying to get the PB booster nuts off from underneath the dash to install a new PB booster. Man, I hate getting under the dash in a C3......

I'll post back when I get her rolling. Thanks to everyone for chipping in!!!!!

Marc in Indy