canomjm1a1

My 76 vette is giving me fits with the brakes! I can't figure what I'm doing wrong! I adjusted the booster pushrod to have a firm pedal! Problem is, about 5 miles down the road, brakes lock up and the pedal is rock solid! The instant fix is to back the master cylinder off the booster! Any ideas or procedures?

MrJlr

Back off the push rod....power brakes should not be that firm.....variants in vacuum will cause that.

Trent Mayo

I have been chasing this exact same problem for three years now (mainly because I was in school and didn't have time to work on it). Its been by far the most frustrating thing I've dealt with on my 75! My rear brakes are the ones that always lock down after about two miles, only to be temporarily fixed by bleeding the rear line at the master cylinder.

Mick71

Make sure there is freeplay between the pushrod and the master cylinder piston when pushing on the pedal. If not, the brakes will be on a tiny bit, getting them hot and compounding the problem.

canomjm1a1

I got a feeling I know what the problem is! Now that it's getting warmer, the problem is getting worse! I had this near same thing happen when a front left caliper froze! Come to find out, it wasn't the caliper, but the brake hose! I felt like I had good brakes, but when I went to stop, it pulled HARD to the left! The brake pedal got hard and the car quit moving! I cooled the brakes with water and all was fine until it happened again a few miles down the road! The brake hose swells on the inside and allows the pressure of the master cylinder to go through! But the caliper can't overcome the restriction of the hose and the brake(s) to stay applied! I believe this to be the problem! The warmer the weather, the softer the hose! Anyone??!!

hugie82

I've seen the hoses swell and get small but never closed off. I would look at the caliper pistons sticking when they get hot. But let us know if the brake line works out for you
Autozone is selling calipers for $68 each if you want to go that way.

calwldlife

did this just start or after master cyl replace?

Mick71

Might as well just get it over with and redo the whole system and be done with it.

canomjm1a1

No, Mick, this a redo in progress! Already done booster and master cylinder! As a tank mechanic with M1A1 and M1A1 SEPS tanks, we are taught that with troubleshooting and and uncertainty, always replace the cheap parts first! Think I'll start with the hoses first and go from there! I'll let ya know!

calwldlife

so when did the issue start, before or after
mc change?

Arkyvette

If your rubber hoses are old or unknown. Replace them. Seen many a caliper hang up due to this issue. The opposite problem of one blowing out is even worse. Your symptoms certainly sound like it could be a hose.

71corvette

Please see my post about brake pushrod length in the general discussion forum. Look for 71corvette. If your brake pushrod is a just a little too long, it won't let your master cylinder valve return all the way back. This in turn causes the master cylinder to pump up the brakes and eventually it will lock either the front or rear brakes. If you have power brakes, I have provided the correct pushrod length. I validated the length I provided tonight by measuring a new power brake booster. If your pushrod length is correct, then your idea on brake hoses could be worth checking especially if it is pulling to one side or the other.