Help! '79 mystery brake problem

[averyed]

I have a ’79 coupe with brake problems. There appear to be no leaks anywhere in the system. The brakes have been bled numerous times, the calipers replaced, and the master cylinder replaced twice in the past few years. Still, within a couple of weeks or months the pedal is down to the floor. Has anyone else had this problem? Is there something else to check that all the mechanics have missed? Thanks in advance for any ideas.

 

[EC_BG124]

Pedal to the floor without any noticable fluid loss is the classic bad master cylinder symptom. Even though it's new it could be bad, or, by chance was it properly bench-bled (on or off car) upon installation. ... If so you're absolutely right... very mysterious (and frustrating I'm sure).

 

[Rex S]

When you say the pedal is down to the floor, is the master cylinder empty or still full? If still full the master cylinder has an internal leak past the piston. If it's empty but no sign of leaks, look in the brake booster as they can store lots of fluid. Many brake parts "rebuilders" leave a lot to be desired.

 

[6880 Mike]

Check the run out on all four corners. Excessive run out can cause the rotors to pump air into the system.

 

[averyed]

I could believe the replacement master cylinder was bad, which is why I have replaced it more than once, but the problem keeps coming back. The odds of several bad parts is not as likely. As EC BG124 says, very mysterious and very frustrating.
I think I'll try one more time this weekend and replace the whole system at once. I will definitely check Rex's and Mike's suggestions. Thanks to you all.

 

[damoroso]

You might also try "power bleeding" if you haven't bled the brakes that way yet. You can buy or make a power bleeder (I made one with a small garden sprayer, some tubing, a plate of steel with a rubber gasket and a C clamp), the idea is to slightly pressurize the master cylinder with brake fluid, then go to each bleeder as normal. Rotor run out and system bleeding seem to be the primary causes of what your seeing.

 

[averyed]

You might also try "power bleeding" if you haven't bled the brakes that way yet. You can buy or make a power bleeder (I made one with a small garden sprayer, some tubing, a plate of steel with a rubber gasket and a C clamp), the idea is to slightly pressurize the master cylinder with brake fluid, then go to each bleeder as normal. Rotor run out and system bleeding seem to be the primary causes of what your seeing.

I haven't tried that, but do have all the parts you mention to make the power bleeder. I'll try it as well...I'll try anything reasonable at this point! Thanks a lot.

 

[bill81vette]

almost sound like your rotors are not running true like Mike has suggested..
excessive run out WILL allow air into the calipers!!!

 

[Vettehead Mikey]

loose/worn wheel bearings or rotor runout will cause this.

 

[averyed]

Thanks to everyone who suggested rotor run out. I haven't heard of this before and will definitely check into it. It seems like a real possibility for a solution.

 

[GTR1999]

Sounds like a runout issue. You bleed the brakes, then a few weeks later you have to do it again. I wrote a whole thread on how to repair this so rather then type it out again do a search or PM and I'll get you the link. The runout should be under 003" for every rotor. VERY common problem with the 65-82's mostly caused by mechanics not experienced with the early corvette brake system. Turning the rotors will mostly likely make them worse unless the lathe operator really understands the system, again not too common.

 

[averyed]

Gary, quick question: your theory of rotor runout seems the most plausible (especially since it's very hard to find any mechanic familiar with C3 brake systems around here). However, shouldn't bleeding the brakes eliminate the problem temporarily? I just bled them again, this time with "speed bleeders" to be sure I wasn't mistakenly introducing air into the system, but the brakes are still spongy. Any suggestions? Thanks....

 

[bill81vette]

If the brakes are still spongy the there is still air in the system.
and it need to be removed!
and yes it does eliminate it temporarily as you said in your 1st post
when you replaced the calipers did you use 'o' ring or lip seals in the calipers???

 

[Vette79]

If the brakes are still spongy the there is still air in the system.
and it need to be removed!
and yes it does eliminate it temporarily as you said in your 1st post
when you replaced the calipers did you use 'o' ring or lip seals in the calipers???

Since I did this last year and learned a few things, I have to ask a couple of questions which hopefully might help. Do you have the correct master cylinder for your year vette? If so, did you bench bleed the master cylinder? Did you bleed them in the correct order also using a rubber mallot tapping on the caliper to release trapped air bubbles? Please see this link from my brake job last year. It may be helpful especially with pics of the master cylinder. There are different MC for automatics and manuals.

http://www.corvetteactioncenter.com/forums/c3-general-discussion/116128-brakes-pressure-gone.html

 

[averyed]

Turns out the master cylinder I just bought in Feb had a leak. So I replaced it yet again, and after another bleed, the brakes are now better. So that problem at least is solved. Rotor runout is back on the table as the most likely underlying problem, but it will be hard to find a local mechanic who can fix this. None so far are aware of it, and replacing or turning the rotors is beyond my skills. In the meantime, thanks to “speed bleeders,” I can keep temporarily fixing the problem a lot quicker and easier and stay safe.

 

[Vette79]

Turns out the master cylinder I just bought in Feb had a leak. So I replaced it yet again, and after another bleed, the brakes are now better. So that problem at least is solved. Rotor runout is back on the table as the most likely underlying problem, but it will be hard to find a local mechanic who can fix this. None so far are aware of it, and replacing or turning the rotors is beyond my skills. In the meantime, thanks to “speed bleeders,” I can keep temporarily fixing the problem a lot quicker and easier and stay safe.

Rotor runout is one of my tasks this spring or early summer as I never got around to doing it last year. I have new rotors and pads as well as the guage to check rotor runout. Hopefully I'm within specs and I'll just replace the pads.