brake problems..

[cleansquare]

Guys. I just wnt thru this with my 85 c20. I had lost rear fluid through a leaky MC and the proportioning valve shut the rears off, and I ended up having to get a new proportioning valve after a new booster and master cylinder didn't fix it. Huge pain in my butt.

Now I'm building the short wheel base 85. I replaced the front calipers, front rubber hoses, and rear rubber hose. I filled up the mc and let it kinda gravity bleed awhile so I wouldn't have to bleed so much later.

(Before I did that, the master cylinder was plugged and I slowly pumped the brakes with th mc cover off to get out any air that had entered when the mc emptied itsekf when I initially removed thr brake components.)

I screwed in all fittings into the calipers, rear axle and everywhere and tested them for snugness. After having my wife pump the brakes a fewtimes it became clear my bleeders in the back were stopped up. I had fluid when I'd crack the line but not in the bleeder. Pulled them and poked a cotter pin in there and voila, out came fluid.

Bled all 4 corners till blue in face. The rear never had more thsn a trickle, EXACTLY what the c20 did with a faulty proportioning valve.

The fronts had about 4" of squirt to them, not near as much as the c20 fronts had which would squirt fluid all the way to the inner fender.

I checked the prop valve little reset button and its not tripped. There NO light in dash indicating an imbalance or that the prop valve is tripped.

One thing that confuses me.

I couldve SWORE on my C20, that the front chamber in the master cylinder went to the front brakes. And the rear chamber to the rear. I coulve also swore that the lines entering the MC were different sizes so you couldn't mix then up.

Well, on this truck, the rear chamber was emptying fast while I bled the front brakes. The front chamber never moved. Nothing was really coming out of the rear anyways. When you took the cap off the MC and watched the holes in the MC while someone pushed the pedal, the rear chamber would squirt high but NOTHING out of the front chamber (rear brakes I think). Absolutely no movement in fluid. Or very very little when it would move.

Is this indicative of a bad MC?

The lines on the truck are thr same size fittings entering the MC. Coukd they have gotten mixed up? Would it even matter if they had?

[cleansquare]

Bump

[Chevy3+3]

I wish I could help but don't know the answer to your question. I'm sure somebody here will help get you fixed up. Good luck

[83GMCK2500]

With rear bleeders closed, I would suggest pumping up brakes and cracking line pre-rubber hose, observe qty/force of fluid. Then I'd remove rear bleeders, disconnect the L & R hard lines from end of new rear hose, slip the ferrule back and put a hose on the hard line for deflection and blow compressed air into each side individually from the bleeder opening. Old hose might've deteriorated and left something in line or in wheel cylinder inlet, objective here to dislodge anything. Would also do it in fluid direction too after reverse direction. I chased spongy brakes on an old F*rd one time, ended up separating the line at every junction available and working my way to MC I blew the lines out, ended up with a firm pedal and solid brakes. Good luck.

[cleansquare]

Thanks for the tips. I gravity bled the fronts and the rear chamber emptied. Bled the rears and thr front chamber emptied.

I think its the MC, there is all new fluid and when you pump it up the fluid in the rear chamber (front brakes) gets dirty as hell and I read that's indicative of bad mc. Also, when vigorously pumping brakes there is not movement in front chamber fluid (rear brakes.)

Headed home with a new mc now. I will bench bleed it first thrn indtall

[Desert1957]

You need to "Bench Bleed" the master cylinder BEFORE installing it on the truck. It will Not gravity bleed itself, air is trapped inside the master cylinder.
see video....

How to - Bench Bleed a Brake Master Cylinder -...

Desert

[cleansquare]

See post #5.....

[cleansquare]

Here's a pic of me bench bleeding the new mc.....

[Desert1957]

OK you bench bled the master cylinder...

1. restricted rear brake hose?
2. brake lines crossed at master cylinder?
3. defective master cylinder?

can't think of anything else , sounds like you checked most commonly found problems.

Desert

[Desert1957]

Don't know if your 85 is like my 87 but here is a picture of the lines at the MC

Note: the larger (front) line going to the MC , don't know what yours looks like when mounted.



Desert

[cleansquare]

Originally Posted by Desert1957

OK you bench bled the master cylinder...

1. restricted rear brake hose?
2. brake lines crossed at master cylinder?
3. defective master cylinder?

can't think of anything else , sounds like you checked most commonly found problems.

Desert

1. Reae hose is new
2. Front line is the bigger line and feeds the rear brakes
3. Mc pictured above is brand new. I think this is what the problem was, its on the truck I just havemt had any help bleeding brakes to see if it fixed it