Brake Help (extremely long)

[Todd 91]

85 Chev Silverado- 1/2 ton if it matters. Changed pads and shoes, had drums and rotors turned. O'Reilleys said that that would be the last turns on each. Started because of low pedal pressure. Still have it. Changed master cylinder. Bled it on the bench. Plugged line holes and pushed plunger in slowly many times until it firmed up and no air came out of bores inside. Bled brakes. R/R, L/R, R/F, then L/F. Couldn't get the left rear to stop bleeding air. The right rear went ok but the left wouldn't stop. Had the tube inside a cup of brake fluid to see the bubbles. Had the Mrs pump the pedal til it firmed and then I cracked the bleeder, let the fluid (or air) stop, closed bleeder, she let off the pedal. Pedal firms up no problem without truck on, but as soon as I turn the key, it dives to the floor. It goes down to where I can get the toe of my shoe under it and that's it. Anyone have any ideas on this one? I was told wheel cylinder but that it'd be leaking. One other thing. I tried the reverse and hit the brake thing to adjust the rears. It seemed to help but the brakes squeled when I was done. Could that be heat build up and rotors? I also adjusted them by turning the adjusters all the way out until I couldn't turn the wheel and then turning them back in until it would just turn. Both sides. Thanks.

 

[zcarczar]

Have you pulled off the rear drums? It is possible that the wheel cylinder is blown and is leaking fluid and letting air in

 

[Todd 91]

I had the drums off to get them turned and also to do the pads. I didn't see any fluid when I had them off. I had them off again last night to adjust and there wasn't any fluid. Is it possible that air could get in and fluid not get out?

 

[zcarczar]

I would think that if the wheel cylinders were blown they would let air in, the only way i know how to tell if they are blown is if the seals on the them are ripped. You might also want to check your brake lines to see if they are in good condition. Other than that i geuss you could keep bleeding the brakes and see if they get any better.

 

[Todd 91]

Would those be the seals inside the cylinder or the ones on the posts(?) on the ends that go bad?

 

[Todd 91]

Disregard that question. I found a breakdown. How much do cylinders cost, average?

 

[m j]

so pedal is fine without vacumm to the booster but goes bad when engine is running supplying vacuum?
perhaps test the booster

 

[Todd 91]

That's the way it feels to me mj. This sounds bad but, I can make the pedal pressure firm by pressing the pedal or pumping it a few times, with the engine off, key off. As soon and I start it, the pedal drops to the floor. Is there a check for the booster? We're talking about the piece aft of the master cylinder right? I'm not sure how the vacuum works on the whole system so maybe this is right the way it's working.

 

[m j]

did the left rear ever get bled completely?
if not, keep at until it is bled before going to the next item.
careful not to drain the master while bleeding or you have to start all over.

 

[Todd 91]

It never did bleed out. I just got mad and couldn't figure why the bleed port about 4 feet away was bleeding fine and this one would keep getting air.

 

[m j]

change that one out
perhaps the seal is toast and will gulp air on you.
unfortunately to diagnose a brake problem without it being bled 100% is not gonna happen.
I would be tempted to just pull it apart and inspect the seals etc but I am not aware if this is your daily driver.
best to have a replacement on hand to be 100% that it will run if you need it.