What could cause brakes to lock-up

[Michael J.]

Re: What could cause brakes to lock-up

Mine has the adjustment under the dash on the pedal rod. But without knowing what it should be, or even being sure if that is the problem, I am hesitant to adjust it.

[Patrick H.]

Re: What could cause brakes to lock-up

Mine has the adjustment under the dash on the pedal rod. But without knowing what it should be, or even being sure if that is the problem, I am hesitant to adjust it.

In addition, why would it suddenly un-adjust, so to speak.

If you had just restored the car, then maybe. In your case, I don't think that it's the rod unless someone snuck into your garage and messed with the car. The only time I had a similar issue was when I replaced a master cylinder. In that case the new MC was not machined correctly, and trying a second MC fixed the problem.

[Mark M.]

Re: What could cause brakes to lock-up

Michael, the adjustment on the rod yoke for brake pedal to booster is given in the GM 1963 corvette shop manual. 64 and 65 have supplements but that adjustment isn't revised in them. The 63 manual on page 5-23 says top of pedal height to floor covering at toe pan is 4 5/8". The pedal basically is adjusted there to get a height. Original 64 power brake car should not have a pedal bumper or return spring. The pedal should be floating where the booster with vacuum would put it . Do you have the original 64 booster? There's a 4 digit number and a date stamped on housing tangs on most original boosters. 64 boosters are unique and may also have been used on some of the early 65 cars that got a J61 credit to use up drum brake parts. The 65 66 and 67 disc system boosters are different parts. The rod to master cylinder in the front of the booster has a specific distance. With master out of way the distance from the housing at the two master mount studs to tip of rod should be 1.2". The manual shows small tolerance here. The hole in the master primary piston would also have to be right. With everyone's good points I hope this helps.

[Joseph S.]

Re: What could cause brakes to lock-up

Mike, I'm just reading this now. If the car was just restored, Bill might be onto something. If the master was sleeved and they missed drilling that relief hole you could have the same issue once again during another long trip. Check to be sure that the hole is free. Joe

[Michael J.]

Re: What could cause brakes to lock-up

Thanks guy, I did run a small wire into and out of the relief hole (as shown in the picture above, BTW in that Z06 MC it is on the right side of the bottom of the reservoir) several times and it is clear. I then refilled the MC with fresh fluid and drove around some without any issues. I am going to assume the restorer (actually two very well known ones worked on the car) knew enough to adjust the rod properly. I will take another long trip soon and keep my fingers crossed, but if it does happen again at least I now know for sure what it is. Thanks for all the help.

[Bob J.]

Re: What could cause brakes to lock-up

hoses probably

I agree.Take an IR gun and check temp at each wheel next time it happens.
Bob Jorjorian

[Mark M.]

Re: What could cause brakes to lock-up

Michael , I looked closer at the 64 supplement and saw ( brake fail to release ) causes for that system. There's also an exploded view of that master cylinder. Part 6 check valves seem unique to that master.

[Michael J.]

Re: What could cause brakes to lock-up

Thanks Mark, I was wondering if those big nut plugs the brake lines go into had some sort of valve in them, it appears they do.

[Dick W.]

Re: What could cause brakes to lock-up

Make sure the rod has free travel next time they lock up. I have seen Saturday night mechanics adjust the rod to the point when the fluid got hot, the brakes would start dragging, the further you went the more heat the brakes would generate, the tighter the brakes got.

[Michael J.]

Re: What could cause brakes to lock-up

Thanks Dick, that is one part I am still worried about. When they did lock up, the brake pedal would not move in or out. I'm not sure that would happen if the port was plugged (??) but maybe so, if not, I will know the thing to adjust next time it happens.

[Mark M.]

Re: What could cause brakes to lock-up

Gentlemen, This is a power brake car. The pedal is adjusted to achieve a height. If a bumper was in it could cause a problem if the pedal was adjusted up too far. Some original 63s have been noted to come with a bumper and possibly return spring and sleeve. You would simply adjust so pedal was off the bumper. 64-66 and 67 cars had non-power or power brake pedal assemblies. Power had no bumper even though the bracket is there and no return spring and sleeve. The pedal to booster is simply adjusted to float to a height off the floor. The front rod on the booster is non adjustable from the factory but service part is adjustable. That distance is important. So would the parts being used in master like the primary piston hole depth. The rod adjustment on non power brake car needs 1/4" play of rod in master while pedal is against bumper from return spring pressure. Too tight here would be a problem. Last adjustment is brake light switch. I double checked on my 64 power brake car. I'm guessing your restorer used a good master sleeving and booster service as I have but kinks pop up to be ironed out. Mike could you post a picture or two of that J56 car? Only 29 made in 64 and I am partial to 64s. Enjoy that toy.

[Bob J.]

Re: What could cause brakes to lock-up

I think it is very easy to check if the adjustment of the pedal is correct.
I've owned a 63 Z06 for the past 40+ years and it's not a hard thing to do.
If Michael removes the pin in the pedal clevis it should be able to be re-installed WITHOUT depressing the pedal.
If you have to push the rod with clevis down coming out of the rear of the PB unit to install the pin it's adjusted wrong. Correct????
Bob Jorjorian

[Mark M.]

Re: What could cause brakes to lock-up

Yes. Well put Bob. This adjustment is simple when understood for power or no power. I'm leaning towards the check valves or booster rod in master distance. Have you any knowledge of the kelsey hayes check valves making a problem?

[Michael J.]

Re: What could cause brakes to lock-up

Thanks Mark and Bob. Bob, I just went out and tried that, and yes I don't have to push the pedal down to reinstall the pin, so guess that part is OK, thanks for that tip. BTW, the hoses are all brand new.

Mark attached are some photos, it is a quite unusual car, and has been sitting in various places around the country in various states of disrepair and awaiting restoration since 1973 or so, and I am sure this was the longest (40-50 mile) trip it has been on under its' own power in 40+ years.

[Mark M.]

Re: What could cause brakes to lock-up

Mike , beautiful 64. It looks like the car in Car and Driver tested back then. I saw the same car and colors in Bloomington 2013. Not to sound too repetitive, but those check valves (residual pressure valves) behind the caps seem to be a part to cause your symptoms if faulty. They let 10% or so pressure to remain in the line normally. All drum cars have them. A bad one may be repairable since yours is so rare. Well I hope it doesn't do it again. You have one of the rarest 64's on the planet. Thanks for pictures