Hey all, so my brakes on my '91 Z28 have been working fine - firm pedal, stops pretty well. But i had the car in the air last week and when idleing the brakes couldn't stop the rear wheels from turning. Since the rear disks and pads are good and the calipers didn't seem to be sticking, I suspected the MC as the culprit. I went and bought a reman MC (dated June '96 but it was the newest one they had!), bench bled it and bled all 4 lines. The problem is that i couldn't get the air out of the lines. It kept coming out in spurts, even after about 20-30 cycles of the brake pedal being pressed air still kept coming out at the valve. The level in the MC was fine and i found no leaks anywhere. I can still drive the car but the brakes are pretty spongy and the pedal travels too far. So my theory is that the MC i just bought and spent the hours putting in may be leaking internally and keeps sucking in more air. Sound right??

 

could be, sometimes it does take a really long time to bleed all the air out though. could it maybe just be the cap on the mc?

 

Wow, you're scaring me. I didn't have the cap fully on when i was bleeding everything, just layed it over the top to keep dirt out of it. Does the cap have to hold pressure for the system to bleed properly? I figured as long as there's enough fluid in the resevour it wouldn't matter if it was tight or not..

 

well, i'm not totally sure but whenever we bleed brakes, we have the cap on tight. i think that might make the difference for you. try it, what the hell? if it does work, you can save on a new mc.

 

I think it would be better to know for sure before the 2-3 hours labor it takes me to bleed the brakes myself.. No charge to exchange for another MC at the parts store, again just a matter of labor time. If i were to try the rebleed with the cap on, do you think I'd have to rebleed the MC again as well?

 

you shouldn't have to bleed the mc again. get someone to pump the brakes for you, it should only take 15 minutes or so.

 

So I rebled the brakes with cap on, same story. Then i cracked the line at the MC and tried to bleed it there and found that the rear most line kept pushing out air. I went back to the parts store and exchanged the MC for another one, put it on and it and the rest of the lines bled fine. Problem solved, but it does suck wasting all that time on a bad MC! (it's still takes a few hours to put on a MC and bleed it even with a helper when you're on jackstands in the street!) I did learn that you don't have to take the wheels off to bleed the brakes though..

thanks Tom!!

 

I wasted a few days on a "rebuilt" m/c myself, too. Damn thing drove me crazy- how could a rebuilt m/c be bad?? Well, it was... what a shame.

TIP: If you remove the snap-ring on your m/c, and see that the inside is corrosion free, you can rebuild your own m/c. It costs a few dollars more than getting a pre-rebuilt one- so why rebuild it yourself? Because you know it's done right. My theory is, that they rebuild these m/c's, then they sit for a year or so in a hot, dry warehouse. We come along, fill it with brake fluid, press on the brake pedal, and Bang! We rip the dried out seals in half. That's just a theory, though.

Shoot, I thought I had the part # for the Bendix rebuild kit lying around. I'll see if I can dig it up tonight.

Oh, hah, Tip #2: Buy the Phoenix Injector (see my message at https://www.thirdgen.org/techbb2/sho...threadid=78725 ), because you can pressure-bleed a master cylinder directly, without even screwing with pushing the blasted piston in! Their video has a plexiglass m/c, and if you follow their procedure, you can see the air leave.