1969 brakes locking up

[D S.]

A friend with a '69 Corvette has been experiencing his power brakes locking up. It started with the two rear brakes but now it's all four. He says he has removed and checked the booster and master cylinder and they seem to be in order. When he began bleeding his brakes he noticed that brake fluid was not getting to the wheels. Do those proportioning valves go bad? He thinks that is the problem.

Thanks,
Scott Sims
Texas Chapter

[Edward J.]

Re: 1969 brakes locking up

Scott, I don't know that I have ever seen a proportion valve cause all 4 wheels to be locking up. what comes to my mind would be a booster issue or the push rod on booster out of adjustment. What can happen to the booster is the internal diaphragm return spring fails(breaks) and this will cause the diaphragm to apply the brakes slightly with the engine running. Truly the best way to test the problem out is have the can together and the 4 wheels hanging and make sure the wheels spin freely and then start the car up and see if the wheels will spin. If not next step would be to back the nuts off on the master cylinder and pull away from booster and see if the wheels then spin.

[Terry M.]

Re: 1969 brakes locking up

Another thing can make the brakes lock up. Has he had recent work on the rotors before this started? If the rotors, all four of them are not true -- that is without run-out -- they will cause the pistons to pump air into the brake lines and lock up the brakes. The Factory Service Manual has instructions for measuring rotor run out. I know it is old fashioned to read books, but sometimes it is necessary.

[Joe L.]

Re: 1969 brakes locking up

A friend with a '69 Corvette has been experiencing his power brakes locking up. It started with the two rear brakes but now it's all four. He says he has removed and checked the booster and master cylinder and they seem to be in order. When he began bleeding his brakes he noticed that brake fluid was not getting to the wheels. Do those proportioning valves go bad? He thinks that is the problem.

Thanks,
Scott Sims
Texas Chapter

Scott-----


There is NO proportioning valve on any 1969 Corvette. What you are referring to is the brake fluid distribution block and brake warning switch. It is highly unlikely that this component could cause the kind of problems you describe.

[Dan B.]

Re: 1969 brakes locking up

"Brake fluid not getting to the wheels"

When were the rubber brake hoses last replaced?? An internally collapsed hose could be the problem.

[Bill M.]

Re: 1969 brakes locking up

I had the brakes on my '65 lock up while I was driving it. I found the small hole to the left in this picture was plugged.



I used a paper clip to clear the hole. Be careful when doing so in case the piston seal is blocking the hole, which means the pedal needs adjustment.

As you're driving the car, the brake fluid will heat up. It expands into the reservoir through that small hole. If it is blocked, the expanding fluid will apply the brakes, which will heat the fluid, and the brakes will apply themselves to full lock-up.

[D S.]

Re: 1969 brakes locking up

Thanks to all for your comments. I forwarded them to the friend who is having the problem. His Corvette seems to be the perfect color for all his dilemmas---(lemon) yellow. If it's not one thing it's another.

Terry, I missed seeing you at the Texas Chapter Regional meet in Frisco. I was there on Friday.

[Michael W.]

Re: 1969 brakes locking up

If the problem persists, have your friend loosen the two nuts that hold the master cylinder to the booster, just a few turns. If that releases the brakes, there's a problem with the linkage between the two. Fairly common problem.

[Terry M.]

Re: 1969 brakes locking up

Terry, I missed seeing you at the Texas Chapter Regional meet in Frisco. I was there on Friday.

I was there Thursday to Sunday. We had the AJS in the classroom Friday morning, but we were with the cars in the afternoon.

[John H.]

Re: 1969 brakes locking up

I had the brakes on my '65 lock up while I was driving it. I found the small hole to the left in this picture was plugged.
I used a paper clip to clear the hole. Be careful when doing so in case the piston seal is blocking the hole, which means the pedal needs adjustment.

As you're driving the car, the brake fluid will heat up. It expands into the reservoir through that small hole. If it is blocked, the expanding fluid will apply the brakes, which will heat the fluid, and the brakes will apply themselves to full lock-up.

In master cylinder-speak, that's the "compensating port"; that tiny hole is where the plant's "evacuate-and-fill" probe seated to (a) Draw a high vacuum in the brake system and (b) Inject brake fluid into the system at 60 psi with the system at high vacuum; we never touched the bleeders.

[Joe L.]

Re: 1969 brakes locking up

In master cylinder-speak, that's the "compensating port"; that tiny hole is where the plant's "evacuate-and-fill" probe seated to (a) Draw a high vacuum in the brake system and (b) Inject brake fluid into the system at 60 psi with the system at high vacuum; we never touched the bleeders.

John-----


Yes, and I wish there was a system like that available for SERVICE use.

[John H.]

Re: 1969 brakes locking up

John-----


Yes, and I wish there was a system like that available for SERVICE use.

Joe -

Yes, that would be nice, but just the fluid pre-processing unit (heated vacuum degassing system) would be about $100,000 today; the evacuate-and-fill is a one-shot process, and can only be done on a new, completely DRY system. Once the system is wet and fails (i.e. has a leak), it's back to conventional bleeding methods.