boblad

While out of town on business driving my 1979
Corvette, exiting the freeway I noticed a loss of braking power suddenly and then the brake light went on. Arriving at my destination, the front rotors were hot to the touch and both rear rotors were cool. The master cylinder was full of fluid in both reservoirs and there was no evidence of leakage anywere. I also noticed that the emergency brake provided no stopping power. I thought that failure of both rear calipers at once was unlikely so am suspecting that the rear brake circuit of the master cylinder failed for some reason. I also suspect that the loss of emergency brake must be unrelated since I can't verify when it last did work since I never have to use it.
any thoughts?

timbo79

Check your proportioning valve.. Light should have come on. Also check blocks in theb lines... Your parkingf breake is a drum brake inside wheel... (hub) Not relying on brake fluid, but rather a cable. These can be adjusted.., I would suspect your cable there is loose o boken, or drums need adjusting.

Matt Gruber

when was the last time the calipers were rebuilt?

boblad

The brake light did come on and is staying on as I would have expected when the pressure became uneven. I suspect it is more likely that the master cylinder is bad than the proportioning valve. Not sure how to troubleshoot the valve so I'm inclined to kit the cylinder first.

boblad

the calipers were rebuilt a couple of years ago as part of a regular brake job.

isosceles

I had similar symptoms. What wa happening is that my rear rotors had too much runout and were doing the notorious air-pumping. When too much air got in the rear system, the porportioning valve blocks it off and diverts all the braking power to the front (seals off the port so no fluid can get to the back because the air in the system simulates a failure)

You can try re-bleeding them. You might want to check rotor run out first. Do a search to see what the max should be.

But to get fluid to the rears you'll have to crack one of the fronts too so that the piston in the porportioning valve re-centers. Once you get a decent flow of fluid going to the rears you can close the front and start the proper bleeding sequence.

boblad

thanks for the great advice. I had that same thing happen a long time ago and had to replace a rotor due to a very slight crack. I'll check the runout first and go from there. cheers

dath

I myself would suspect one of your rear calipers is having issues, either due to runout as mentioned, and/or due to pitting/wear in the cylinders. I am in the process of replacing all four calipers on my car with stainless steel sleeved ones. I previously tried putting on two "rebuilt" calipers on the rear and these started ingesting air again. I just took one apart yesterday to learn how to do it on a junk one and I noticed something that makes me very angry. :mad Turns out that my freshly rebuilt caliper (probably less than 1000 miles on it) had pitting in the cylinders. Long story short, buy the sleeved calipers and do not trust Napa rebuilds...

A couple more things to be aware of while you are doing this... The rotor and spindle are turned at the factory as a unit. So if you decided to replace the rotors, you may need to shim the new rotors to eliminate runout, even if the rotors themselves are true. Also, it has been suggested by many folks that the design of the Corvette calipers with springs behind the pistons may result in making the air sucking problem worse because the pistons constantly try to reseat against an out of true rotor. The solution to this is to make sure your rotors have less than .03" runout when installed *AND* to remove the springs from your calipers. Also, with the sleeved calipers, you can get either lip seal, or o-ring seal, something I really wish I knew when I bought my sleeved calipers as I thought all sleeved calipers were o-ring... Anyhow, it is said that the o-ring design is superior as well. Probably since an o-ring isn't as directional is a lip-seal it helps when you have runout pushing the pistons against the normal direction of travel...