When I bought my 78 about 6 months a go, one of the things I noticed first was new calipers and lines. The brakes were spongey, but my only other experience was with an 84, so I assumed that was just the C3 brakes. The first week in June I lost braking totally. No primary brakes and no emergency brakes. Since brakes are one of the things I refuse to work on, I took the car to a local repair shop for repair. They replaced the front brake connector lines, saying that one of them was causeing the left and right brakes to get uneven pressure. That sounded possible, and the charges were reasonable, so I took them at their word. Two days later, no brakes again, this time they said the master cylinder was bad. Even though they only charged for the master cylinder, my faith in them was strained. Two days latter, no brakes again. The shop said the rebuilt master cylinder must have been defective, return it for replacement. They replaced it with another rebuilt master cylinder, but three days latter, no brakes again. This time they said that maybe the NAPA rebuilts were no good and maybe we should up grade to a new master cylinder. So they charged me for the difference in a rebuilt and a new master cylinder ($109) and installed the new one. After only one day, you got it, the brakes went out again. Does anyone have a clue what the solution is? Seems like they are only addressing the symptoms. :confused:

 

Not sure this place knows much about C3 braking systems, and I may not be an expert either, but suggest you may have a rotor runnout problem, which frequently is indicated by bleeding brakes, till you have good hard pedal, and then the brakes go away, pedal gets spongy, after driving a while.

Rotor runnout means that the rotors are moving in and out as they are turning, and this in / out movement results in air getting pumped into the brake hydraulic system through the in / out action of the brake caliper pistons.
I understand this can occur even though your brake caliper pistons are not leaking any brake fluid.

Suggest you take vette to somebody familiar with working on C3 brake systems and have them check for this runnout condition.

 

I've had my 78 for nearly 15 years and have worked on the brakes a lot. I autocross my car so I'm careful about the brakes. One sure sign of a bad master cylinder is that the brake pedal fades while you sit at a red light or stop sign but you never lose any fluid. You didn't say if you are losing fluid or not. I would think if your runout is bad enough to cause you to lose brakes in a day or two you should be able to feel the vibration in the pedal. I think I would look for a leaky caliper. Good luck

John

 

Thanks for the response. I am not losing any fluid. the mechanics say the master cylinder is leaking internally from chamber to chamber. I thought it may be caused by a sticking caliper piston that increases the back pressure until the master cylinder.

 

Did they bench bleed the new master cylinder before they installed it?

 

Don't have any idea. :confused:

 

If you can pump your brakes up, then they just didn't get all of the air out of the system. Sometimes you have to bleed your brakes several times to get all of the air out. I don't think the fluid can leak from one chamber to the other in the master cylinder, I believe they are two separate systems. The only 'wild card' I can think of is the proportioning valve...I'm just not sure about that.

John

 

How old are these mechanics? If they are 20 something and don't know C-3's you may want to take it elsewhere.

 

A very common problem! You need to check for play in the wheel bearings and check the rotor run-out. The rotor run-out spec is .06" and if it's a little more than that the pistons will be moving in and out of the bores too much because it's a fixed caliper. The standard factory-style lip-seal calipers are known for cavitating air in the system due to the stock piston seal design. Also, if the car sits the flimsy lip-seal will break down just from the weight of the piston. Thats why we offer a patened o-ring design which will eliminate the lip-seal problems all together. I recommend the o-ring conversion kits which come with all the seals and dust-boots to rebuild the whole caliper. They work better if you have stainless sleeves in your calipers already or you can order the caliper already built with a lifetime warranty. :flag