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I am working on my 1970 Corvette non-power brake car. After much work, I keep having the same problem. The passenger rear brake locks up in hard braking. I have bled the brakes all possible ways..pumping the pedal, pressure bleeding, and raising the rear of the car and getting air out of the master cylinder. Parts that I have replaced and fixed: all new rotors, rebuilt all 4 calipers, new soft lines, new proportioning valve, and new master cylinder. I also disconnected the front brake lines and blew them out with air. I am using DOT4 brake fluid. I have road tested the car on short test rides multiple times and each time the passenger rear locks. On one test both rear brakes locked and only once have all four locked. The locking is inconsistent and not predictable in reflecting the work done.
When you say that you replaced the soft lines, do you mean that you replaced the brake hoses? Old brake hoses can come apart inside and act like a check valve and keep the pressure on. Lou.
I feel your pain...... Has the proportioning valve been changed to the wrong valve? The 1970 doesn't have a normal proportioning valve. What front and rear calipers do you have? The front calipers should be stronger than the back calipers. If the front and rear calipers are the same, this can cause rear lockup. Brake lock up can be caused by contamination. Is there anything getting into the caliper/rotor.
Mine does that too under extreme braking.
I've replaced the whole system, except for the hard lines, and it still does it.
Maybe that wheel has less weight on it?
It may be the nature of the beast?
I don't think that there is any adjustment.
Maybe remove a little material from the pads?
I had a '62 and one of the rear linings got contaminated with brake fluid from a leaking wheel cylinder. That wheel would lock up when the brakes got hot.
All four calipers are stainless steel lined. Two front calipers are SSBC. Passenger rear is DELCO REMY and driver side rear is SSBC also. Proportioning valve was purchased at Topflight. Brake pads are not contaminated with fluid. Thanks for responses, Laz
Top flight sells many different proportioning valves. If it looks like this, it's correct. You have two different calipers on the rear. Are they the same, just different sellers?Maybe one has more stopping power. Technically this isn't a proportioning valve, it's a distribution block. It's only function is to light up your brake warning light.
That's good. I see a lot of people trying to "improve" their brake system and install one of the fancy new valves and make a mess of their system. I would take another look at the type of calipers, front vs rear, and rear left vs right. The early C3 used different calipers, front to rear to bias the braking. On later C3's this was accomplished with the fancy proportioning valve.
I don't think adding a proportioning valve would help.
You have 1 line going to the rear brakes, and it T's off to both.
The valve would have to be in front of or between the 2 calipers.
Having 2 different brand calipers on your rear wheels may be the problem. It may be worth the effort to change it to match the other side, as a last ditch response.
Also, the hard line from the T to the wheel that doesn't lock up may be gunked up inside, inhibiting flow??
Or, the T itself??
Grasping at straws here. I feel your pain.
It is possible that the shuttle valve inside your distribution block is binding up inside and causing rear brakes to hold under heavy apply force. It is also possible that your parking brake shoes are somehow being applied under heavy braking (don't know how that would happen?) and causing binding. You state that ALL of the flex hoses going to the calipers have been changed, correct? Because that would be my #1 pick for the cause of your problem.
Anything is possible at this point. Flex hoses are all new. I do have a mismatched caliper that has stainless steel pistons in the rear passenger wheel that is prematurely locking under heavy breaking. I am going to replace those pistons with aluminum one. That is my next step to try and rectify the situation. Will keep you posted. Thanks for all replies, Laz
If it is an original distribution block for the 70,,,, it doesn't reset. It is only a switch that lights the light. It doesn't get stuck in the wrong position. Stomping on the brake pedal doesn't do anything to this valve. It is like stomping on the brakes to reset the brake light,,,, it doesn't happen.
Yes, the dist block shuttle valve WILL reset, if it is 'tripped' in one direction and any leaks have been repaired. It isn't a guarantee that the valve will reset, but that is the FIRST order of business to get it back to center position. Just give it a good, hard, stomp to attempt a reset.
P.S. If it has been tripped, the "BRAKE" warning light in the speedometer should be illuminated (if the bulb is good and system is operational).
If this type of valve is tripped, it doesn't shut off fluid to the side that is leaking, it only makes electrical contact to light your brake warning light. A tripped valve doesn't do anything to the flow of the fluid. Now, if it's rusted all to hell and has corrosion and gunk in it,,, it could be plugged up, as any brake part can be. If the valve is functioning normal the springs will center it.
Actually, under pressure, the spring at the end of the shuttle valve will compress and it will nearly eliminate flow to the defective 'half' of the brake circuit. That's its purpose: to let you know that there is a problem (with the BRAKE light). to allow the continued use of 1/2 of the brake system, and to prevent the loss of fluid from the defective side of it.
Actually, under pressure, the spring at the end of the shuttle valve will compress and it will nearly eliminate flow to the defective 'half' of the brake circuit. That's its purpose: to let you know that there is a problem (with the BRAKE light). to allow the continued use of 1/2 of the brake system, and to prevent the loss of fluid from the defective side of it.
I am sorry,,,,, but that doesn't happen and that's not the purpose the valve. It only lights the warning light. The MC is divided in two halves. Front and rear. This is supposed to keep one half working, but I am not convinced of this either. As it says the light will only be on when you are pushing on the pedal,,, and goes off, the valve centers, when you lift the pedal,, no resetting, no stomping.
Don't believe everything you read in a manual from almost 50 yrs ago.
Change oil every 3,000 miles.
Hook distributor vacuum to Ported / Timed port.
Use nothing but Delco parts and GM approved fluids.
Brake warning light will not remain on when the brakes are released.
Open throttle fully during compression test.
Last edited by HeadsU.P.; Aug 28, 2021 at 08:25 PM.
Brake help
