brake problems.....

[1low4x4]

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?

 

[1low4x4]

Bump

 

[da_raabi]

On my C30 the rear (large) chamber is for the front brakes, the front (small) chamber is for the rear brakes. If I remember correctly, yes the sizes were different. Now remember, a lot more fluid/pressure flows to the front brakes, so you will see a lot of fluid movement and bubbling from the front brakes/rear chamber and not as much for the rears. A way to check this is to try to flush fluid through the rears and see if the front chamber empties.

Now when you had the whole thing apart, did you prime the M/C before reinstalling?

 

[1low4x4]

On my C30 the rear (large) chamber is for the front brakes, the front (small) chamber is for the rear brakes. If I remember correctly, yes the sizes were different. Now remember, a lot more fluid/pressure flows to the front brakes, so you will see a lot of fluid movement and bubbling from the front brakes/rear chamber and not as much for the rears. A way to check this is to try to flush fluid through the rears and see if the front chamber empties.

Now when you had the whole thing apart, did you prime the M/C before reinstalling?

Ok. Just checked the c10 that I'm working on, and the line fittings are the same size. However the front line that you are thinking goes to be rear brakes is a bigger lin than the rear line on the rear chamber. This would make sense, seeing as how the fluid has a longer way to go. I also checked my C 20 before leaving today and it is the same way, fittings are the same size but line is bigger in front chamber of master cylinder. I never had the master cylinder removed from the truck, but it was empty from gravity bleeding the fluid out. What I did do is put plastic plugs were the lines would go out of the master cylinder, fill up the master cylinder with fluid and slowly pump the breaks until I saw no bubbles. That to me was the same as installing the plugs with the lines going into the fluid again basically recirculating fluid

 

[1low4x4]

I opened up the front bleeders and let it gravity believe, and the rear chamber in the master cylinder went down to minimum level. Then I close them and opened up the rear bleeders, and the front chamber went down. I blend in for a good bit. Gravity blade. I think it is the master cylinder that is bad, because last night there was all new fluid in the master cylinder and lines, and when we pumps the master cylinder vigorously the fluid in the rear chamber, the one that feeds the front brakes, got really really dirty. I read that this is an indication of bad cups and seals in the master cylinder. And also we put the cap on and pump the crap out of the brakes with all the bleeders closed and the front chamber, which feeds the rear brakes, never had any movement in the fluid, while the rear chamber was going like crazy.

 

[da_raabi]

Yea sounds like a new M/C would not be a bad idea. Its easy enough to do. It also sounds like you are getting a lot of air in your lines from somewhere (possibly the m/c) hence the large activity when you pump the brakes. There should not really be much bubbling in the m/c when you pump the brakes.

 

[1low4x4]

Its not bubbling per say, its a stream of clear fluid

 

[chengny]

Its not bubbling per say, its a stream of clear fluid

That's normal - to get little "eruptions" of brake fluid in the back chamber when the brake pedal is depressed.

Here is an image of the proportioning valve showing the sizes of the connections and where they come from/ go to.

 

[towman]

might think i am crazy but smack the proportioning valve with a rubber mallet when trying to bleed if it is stuck usualy will jarr it loose and allow it to reset

 

[chengny]

Or you could pull the switch body out and use an awl (or a small screwdriver) to shift the spool back to the center.

 

[1low4x4]

Its not the prop valve thats the problem

 

[chengny]

Its not the prop valve thats the problem

Sorry but it's kind of vague what you are asking. I am assuming that you are getting either no flow or minimal flow out of the front chamber of the master, across the P/C valve, down the rear lines and out of the bleeders - correct?

You most likely still have more crap in the lines and it is still plugging up the bleeders.

The cross-drilled holes in the bottom of the brake bleeders are really tiny. Also the 90 degree turn causes any entrained debris to fetch up and obstruct the flow up into the main bleed bores. These show a cross sectional view of a bleeder - they're the self bleed type but other than the spring and ball check, its the same:

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Just because you cleared the bleeders once doesn't mean they are going to stay that way. You're going to have to thoroughly flush the entire rear hydraulic circuit:

1. Have a couple of quarts of brake fluid on hand.

2. Pull the bleeders out of both wheel cylinders.

3. Flush the lines by pumping the brakes. Do this until only clear fluid issues from the bleed holes.

Obviously, have an assistant constantly add brake fluid to the master while you pump the brakes. This will prevent you from getting air in the lines while flushing. If the master never sucks in any air during the flush you will have a much easier time doing the final bleed.

This is not so obvious - while pumping the brakes with the bleeders pulled, go wicked slow!

Having the bleeders out is the equivalent of loosing a brake line. If you pump to hard/fast, the P/C valve will shift over and cut off the flow to the rear brakes. It's not the end of the world but you'll have to stop the flush and recenter the spool.

When you feel that the lines are flushed properly, replace the bleeders (be sure and clear the drilled holes first) and then purge the minimal bit of air that remains in the wheel cylinders.

 

[1low4x4]

Thanks for the advice but I installed the new mc and haven't had a chwnce to bleed yet. I'm sure the rear lines will need to be flushed but I haven't even had a chance to touch it