I have a problem with the brakes on my 1980 Vette that's driving me nuts! This weekend I pulled a rear caliper to replace a u-joint --- I didn't disconnect the brake line. When I finished with thej ob, I had to pump-up the brakes -- as expected. The "brake" light came on briefly, but went out as soon as I got a good "pedal". However, when I started the car the next morning, the "pedal" was very low and soft --- and the "brake" light came on again. I was unable to get the brakes to pump-up, so I decided to bleed the system. I didn't really get any air out of the system, but I couldn't get any fluid to the rear calipers. After having my son "stomp" the brake pedal a few time, I was able to get fluid to the rear and the pedal height and firmness were OK. However, when I checked the brakes an hour later, the problem had returned. So, I assumed the master cylinder might be bad. I replaced the master cylinder -- bled the system and congratulated myself for another job well done. But --- the next morning, the problem was back again!!!

I've verified that rear brakes are the ones with the problem -- absolutely no fluid is gets to the calipers until the brake pedal is vigorously stomped a few times and a typical bleeding process is followed (but I've never actually gotten any air to come out of the system) I've also checked all of the brake lines, hoses and calipers to ensure that I don't have any leaks.

I understand how the proportioning/combination valve functions, but I'm not real sure how a malfunction could cause the symptoms I'm experiencing. I've checked around with a lot of folks, but so far I haven't found anyone who has ever heard of a proportioning valve malfunctioning.

I'm sure interested in any opinions anyone here might have. Is my proportioning valve bad or do I have I overlooked some other simple cause?

Thanks in advance

 

I am new on this site but I have been an owner of a C3 for a while........this sounds like the exact problem I had and it drove me crazy for a long time. A friend of mine who is a shop foreman at a local Corvette dealership told me to change the back callipers as they were allowing air into the system......thus the soft pedal......I disagreeded..... if air got in.......fluid had to leak..... ..needless to say I was wrong........I changed the back calipers on my '82 and problem has been solved now for over two years. I did not change the proportioning valve but I changed the master cylinder which was needless also. His theory was also proven by the pistons in the calipers being slightly damp with brake fluid.
So good luck in your challange and hope this helps.

 

Also consider bad rubber lines, or kinked rubber lines...

 

I have had my Vette for a long time and I have had both of the problems with my brakes. I had a bad metering valve and then this year I had a caliper that didn't leak but let air in. The bad metering valve, proportionate valve, happened about ten years ago. I had changed the whole brake system and I couldn't get a hard pedal. I bled about 2 gallons of fluid through the system. I broke a hand vacummm brake bleeder. At the time you could not buy a metering valve, I got one at the junkyard for 5.00 . As soon as I replaced the valve the brakes were perfect.
This spring I had to bleed my brakes every week. I couldn't find any leaks. I changed the mastercylinder and nothing changed. I replaced the front calipers and now my brakes work well.

 

Thanks for the info. I'm pretty sure that the brake hoses are OK, but I'm going to change them anyway -- they look like they are original and replacing them will at least elimininate another "possibility".

The info about the possibility of the caliper allowing air to enter the system, but not let fluid escape is interesting. Since my symptoms seemed to start after I disturbed the caliper pistons (by pushing them all the way in) I think it's reasonable to assume that the caliper is the root of the problem. So --- I think I'll try replacing the rear calipers before I replace the proportioning valve.

Here's something else I've noticed on my right rear caliper when the brakes are acting up: The pistons are retracted into the bore a lot, IE there is approximately a 1/8" gap between the pads and the rotor. Does this condition indicate anything?

 

Corvette brakes have the pads in constant contact with the rotor as to provide immediate repsonse when the brake pedal is pushed. With that in mind, I would assume your right rear caliper is the culprit of your problems.

Hope you get it fixed soon. I dealt with the same problem for three years.

 

I've had a proportioning valve go bad, symptom was that the front brakes would lock up tight the first time the pedal was pushed, and stay that way. Dont know if this helps or not, but is what happened to me.

JimK

 

I have run into the problem of bleeding back brakes on 3 different cars. I talked to the local mechanic and he told me those proportioning valves don't let the fluid through. In one case I was removing the drive shaft and no matter how hard you stepped on the peddle the rear end still turned. In all cases I removed completely the rear proportioning valves and my rear braking is excellent. No more bleeding problems and if you loose one set of brakes the car still stops just about the same as ever. When I hit the brakes the vet feels like someone grabbed the back bumper (if I had a back bumper). It is the best thing I did for my brakes. I never drive in the rain so maybe for a daily driver it might be a bit extreme but for that dry weather car only it sure wakes up the brakes.

 

Norwalwilhelm

Did you have a good firm pedal when your proportioning valve(s) went bad? I have a very low, soft pedal.

Also, what did you have to do to "remove the rear proportioning valve"? Is it as simple as unscrewing the rear part of the proportioning valve -- remove the "guts", and put it all back together? If it's that simple, I might try it as a test.

 

My peddle felt fine in all cases but bleeding was a pain in the leg. It took toooo much effort to get anything out of the back bleeders. Since I run a line lock I ran a line directly from the back of the master cylinder to the rear brakes splicing it where the proportioning valve disconnects and for the front ran a short line to the line lock then 2 seperate lines from the line lock to the front brakes. In all three cases the braking really improved and no more problems with bleeding the rear. Even if you disconnect the rears completely and go for a drive with open bleeders on the back the peddle will feel fine and you will stop. Try that with the proportioning valve hooked up an back bleeders open.

 

Thanks for all the input. I'm going to be out of town for the long weekend, but I'll start working on the braking system when I get back on Tuesday. With all this good advice, I don't see any reason why I shouldn't be able to get this thing to stop.

I'll post an update and let everyone know what fixed the problem.

Thanks again --- I've only asked a few questions on this board, but I've really been impressed with the technical quality of the answers I've received.

 

Well, I replaced the rear calipers and the brake problem is fixed!!!!

I have to admit that I was very skeptical that my problem could be caused by a caliper that would allow air could leak in, but not allow fluid to leak out . However I'll be the first to admit that I was wrong.

I would like to add that when I removed the pads on one of the calipers I did notice a VERY slight amount of fluid around the edge of one of the pistons. It was so slight that I wouldn't have noticed it if I hadn't specifically be looking for it.

Again, thanks for the all the input I received, both on this board and via e-mail. Without the help I received here, I would have never thought to replace the calipers.