Hi everyone.
This is my first post. I’m looking for some advise and some feedback on my current issues with my 78 Vette. I bought it last January and have basically replaced all of the calipers, rotors and more recently the master cylinder and now the proportioning valve.
It lasted a few good days before the pedal went to the floor and the brake light came back on. I’m at a loss at what do next. The master cylinder is still full.
I’m pulling my hair out.

 

Bleed all the brakes again and carefully go for a ride.
If the pedal goes to the floor again check the rear run-out.
Here's how:
https://www.youtube.com/watch?v=SSxC...ature=youtu.be

 

Need a lot more history on the bleeding procedures.

What type did you use? Pressure, vacuum, gravity, pedal?

What was wrong with the Prop V?

Was the master bench bleed properly?

 

Thanks for the replies. I have to admit I’m a novice when it comes to mechanics but I’m am trying to learn. I needed a friend who works at a shop to help me out with the brakes because I don’t have the Proper equipment to work on it at home.
I’ll give you a bit of a back story in regards to the car. I bought the car at an auction. It had some mechanical issues because it sat for so long ( I’m guessing around 5 years). Replaced the alternator, radiator, fuel pump and a few other small mechanical things. The brake light came on shortly after firing it up and was suggested to me to redo the brakes. I decided to replace all the callipers and rotors and had a friend who worked at a shop do them for me.
This is when my nightmares began. First. One of the Brand new callipers had to be replaced countless ( maybe 6 or 7) times because it kept leaking. The supplier kept sending defective or substandard parts. I could write a book about this gong show.
I finally got things to stop leaking and thought our problems were behind us. That’s when the current issues started. It was suggested to me to replace the master cylinder. So I did that. Drove great for a few days and then slowly the pedal would go to the floor but this time if I stomped on the pedal, it would slightly build pressure and the light would go out but if if I slowly and lightly pressed on the pedal, it would go to the floor and the light would come back on. It was then suggested that it was my proportioning valve that needed to be replaced.
I don’t know if I’m getting bad advice or what but I was actually feeling pretty optimistic after the proportioning valve replacement. The brakes had never felt that firm before and it lasted for a bit longer but alas my hopes were dashed when driving it yesterday and the pedal once again slowly went to the floor and the brake light came on again. This is now where I am. Deflated but optimistic that I can find some answers on this forum. To be honest my friend and his shop have been very good at helping me try and find a solution but I think it comes down to the fact that it’s a 42 year old car and not a lot of knowledge from the masses when it comes to working on these cars.
I hope this sheds some light on the issue.

 

In your first post you said a new master. So lets start with that. Purchasing the wrong master can effect the booster rod to MC clearance gap. Some examples would be the wrong year MC (one size does not fit all).
Or an aftermarket unit.
Or a manual MC fitted to a PB car. (vice versa)
That booster rod gap can be checked with a special tool if you suspect somethings amiss there.

A Proportioning Valve seldom fails. Basically one moving part, brass so no rust. And pretty much fool-proof. But they could go bad. Usually the failure is that the Prop V. closes off the rear brake supply line. In most cases, a quick jab of the brake pedal resets the little piston to center, dash light goes out. But only if there are no open lines (leaks) anywhere.

To isolate the MC as a potential culprit in this issue, you will need to fabricate two test lines. Short, steel, pre-made brake lines can be had at NAPA with the end fitting that fits the MC front port & rear port, which are not the same. Figure out a way close off the other end and bend upwards. With the vehicle brake lines removed, install your test lines. You should be able to stand on the pedal w/o movement. If not, there's the problem. If you can, look elsewhere. Never use a bolt to plug the ports. You will damage the flare inside.

I suspect that some bubbles are working there way up to the MC. That's why the delay in a poor pedal.
For some tips on "Bench Bleeding", click on my profile > photo album > bleeding.

 

Thanks Heads UP. I’ll try your advice. We’ll see if that helps isolate the problem.

 

6 or 7 times on one caliper? I wouldn't care if the replacements were free under warranty, your time has value.

I've had luck with Lonestar-rebuilt stainless-sleeved calipers on original Delco Moraine cores (available from Zip, and elsewhere), using O-rings. I've also had luck rebuilding them myself with CSSB Inc. kits (perhaps an eBay store only at this point).

That said, before you waste another dime or minute, check the runout on that rotor. There are lots of threads on how to do it. Here is one of the best, from @GTR1999, that shows how too check with a $50- tool, and how to correct the problem with shims. You can also turn the rotors with a brake lathe, but risk losing rotor thickness. Unlike single floating piston brakes common to most cars (and possibly familiar to the friend helping you with this), C3 brakes have 4 pistons and are fixed in place relative to the rotor. They require nearly zero runout to function correctly, though O-rings seem to handle a bit more runout than the stock lip seals.

https://www.digitalcorvettes.com/thr...dial-in.77063/

FWIW, both of my rear caliper mounts were broken and replaced when I rebuilt the trailing arms. One bent, one cracked, so you may want to look for that too.

 

anyone have a source for the shims?