OK, I have a 71 GS front disc/rear drum. Replaced master cylinder (actually got a bad one that had a bad piston the first time). Got a new one, bench bled it and started to try and bleed the rears. It seemed like there was little to no pressure when bleeding the usual way. The fronts bled with no issues, the rears had no air, but also did not have much pressure, more like a steady trickle. Also the pedal seems very low and almost goes to the floor when attempting a firm stop. There is some adjustment in the pedal assembly where it looks like you could thread it out and thus provide more travel at the rod/master pipston, but I'm at a loss. HELP!!!
Have you adjusted the rears before you started the bleeding? I don't think rears take near as much volume and they also have a residual valve in the proportioning valve.
If the '71 has a proportioning valve you need to crack it open by depressing the button on it first; keeping open during the bleeding procedure.
OK I was wondering about that! So it needs to be depressed the entire time the rears are being bled? So it's a three man job then? Thanks!!!!!!
Did you get a brand new master or a re-manufactured? If its a reman, I'd bet you have another bad one. Personally, Ive never had to mess with the prop valve when bleeding the rear brakes on any car. Crack the line at the master and see of you have pressure right out of the master. If you have pressure there, try cracking the line going to the flexible rubber hose at the rear. Its not uncommon for those hoses to collapse internally and not allow fluid to flow.
I also had the same issue with the rear brakes trying to bleed them. What I did was let gavity do the work. I placed a bottle with a line from the right rear bleader screw and kept it open and within a day it started to show some fluid in the bottle. This is when I bleed the rear brakes with no problem. George
For what I paid I suspect it is a re-man master. I don't think the second one is bad, it bench bled perfect and the fronts bled like a dream. The reason that I suspect the prop valve is only because a couple of times I had held the valve closed had my Dad pumped the brakes and we still managed to have good pressure when cracking the bleeder, subsequent attempts resulted in a trickle flow.
Allow me to clear some mis-information that I'm responsible for. According to the '70 chassis service manual section 50-23 the disc brake hold off valve (what we've called the proportioning valve) only needs to be depressed for front disc brake bleeding. I usually slip some shims in there or make a kid hold it open. Doing this job once a decade, it is easy to forget all the steps. Always consult some trusted written material.
MAN, I wish you hadn't told me that! ou: I thought for sure that was the problem. The fronts bled fine it's the rears that seem to have an issue. So what IF the front were bled to good pressure and the rears did not as I now have with a weak pedal feel almost at the floor???? Like I said the new master bench bled good, system was in good working order prior, just had a weepy seal that I didn't want running down my booster. What about the push rod, what should I be looking for? Thanks
70 is different from 71/72 70 has the residual pressure valve under the MC and a distribution block on the frame. 71/72 has a combination valve on the frame. Unless you had a line brake, either front or rear, you should NOT need to reset the combination valve. I also have never had to touch one to bleed rear brakes. If the seal was only weeping you don't need to bleed the wheels, bench bleed is all you need. The pushrod didn't change lengths from changing the MC. Remove the rear bleeder screws and clean them out with a small drill bit. Also run a small drill (twisted by fingers only) or wire into the wheel cylinders to be sure the passages are open.
Hold your finger on the rubber boot on front of combination valve with some pressure. Have someone push pedal down. If you feel the metering pin push out the backs don't have the pressure to bleed. It well have to be held in while bleeding ( I use a C-clamp ).
I did take the rubber boot off the front of the combination valve and held it down, it did push up with pressure when my helper pumped the brakes. I will use a "C" clamp on the valve to keep it depressed and attempt to bleed the rears. in the next day or two. Thanks again guys!!!!!!
Well I got a "C" clamp on the combination vavle and bled the rear brakes. Now the rears had good pressure and bled out like it should, pushed a good volume and the pedal lowered as the fluid was bled out. I'm thinking that I got it licked finally. I decieded to bled the fronts to make sure that I'm 100%, I took off the "C" clamp and the fronts bled poorly like the rears previously did. The fronts now have low pressure, tricking out without real good prressure behiend them. The pedal doesn't have huge travel, but it still feels soft to me and goes almost completly to the floor. I can't figure this one out so with that said I ordered a new prop/combination valve from OPGI in the hopes that the issue lies there, no other possibility in my mind???????????? opinions please!!!!!!!!!!!!
By holding pin in it equalizes pressure frt. & rear. Bleed LT frt. 1st then RT frt. then the rear LT then RT. Close system. Remove clamp. Try brakes.
