[Carmine]

Hi everyone. I started a new thread re. my brakes because I think there might be another issue other then vacuum.
My '71 GTO has disc on the front, drums on the rear. I just changed the two rubber front brake lines and the one rear rubber line. I started bleeding the lines from the furthest wheel away from the MC. The rear wheels bled fine with air coming out of the line. Likewise for the front. Went over them several times each, until just fluid came out. The problem is, I can't get the rear brakes to work. When you push on the brake pedal, I try to move the front disc and can't. They are tight like they should be. Not so for the rears. While pumping up pressure, you can't even hear the brakes shoes engage the drum. No sounds of the springs, squealing, nothing. No movement at all. Under pressure the wheels still turn freely.
When bleeding the front lines, the pedal would go all the way to the floor when loosening the bleeder. Not so with the rear drums. The pedal would stop about 2" or so from the floor and go no further when the bleeder was released on each. Fluid would come out, but the pedal would not move. It stayed off the floor. My friend who was helping me said he had both feet on the pedal and couldn't get it to budge, to the floor, when the bleeders were open. Kind of puzzles me. Fluid to the brake cylinders but when under pressure, fluid will come out, but pedal doesn't touch the floor. We did some more bleeding and nothing else really changed. Didn't see/hear any additional air come out of the lines. So, not knowing what else to do, we stopped. I did some research but can't seem to find a problem like mine, so that's why I'm asking what to do/try next?? Any ideas/suggestions would be appreciated. BTW, always had enough brake fluid in the MC. Thank you, Carmine.

[chemnick]

Maybe your isolation valve has switched to isolate the rear brakes? What you're doing may simulate a rear brake fluid leak. Since you said you started bleeding in the rear first I thought this might be a possibility....................................

Chemnick

[NEW64OWNER]

Originally Posted by chemnick

Maybe your isolation valve has switched to isolate the rear brakes? What you're doing may simulate a rear brake fluid leak. Since you said you started bleeding in the rear first I thought this might be a possibility....................................

Chemnick

That's what I would check first. If the piston in the valve moved to shut off the rear circuit you can re-center by opening one bleeder on the front and one on the rear then press pedal to bleed then close both valves and resume bleeding process.

[Carmine]

Originally Posted by NEW64OWNER

That's what I would check first. If the piston in the valve moved to shut off the rear circuit you can re-center by opening one bleeder on the front and one on the rear then press pedal to bleed then close both valves and resume bleeding process.

OK. I'll give it a try. Was I wrong for the way I bled the brakes?? I always thought you started with the wheel furthest away from the master cylinder?? Thanks, Carmine.

[NEW64OWNER]

That's correct Carmine. But on newer split systems it doesn't matter if you do front or rear first but do the one that's farther away in that section (front vs rear)

[Carmine]

Originally Posted by NEW64OWNER

That's what I would check first. If the piston in the valve moved to shut off the rear circuit you can re-center by opening one bleeder on the front and one on the rear then press pedal to bleed then close both valves and resume bleeding process.

I just tried this with negative results. Same thing as before. I even started in the front first and then the rear. Didn't work. I tried the rear first and then the front. Still a no go. The pedal would go all the way to the floor when the front bleeders were opened. When the back bleeders were opened, the pedal stopped about 2" or so from the floor. Just like before. Rear wheels turned freely with pressure. Front disc wouldn't budge under pressure. The fluid seemed to run out faster from the front bleeders. The rear didn't dribble out but doesn't appear to have the same pressure behind it. Is there a Plan B?? Is the proportioning valve bad?? I think it is the original one. Thank you, Carmine.

[NEW64OWNER]

Is the "brake" light on? If the valve moves the light should come on. Bleeding brakes will often move the valve off center due to unequal pressure in the brake line at the valve. Opening 1 front and 1 rear bleeder at the same time then pushing pedal a few times as if you were trying to bleed the brakes will usually re-center the valve as this creates equal pressure on either side of the valves sliding piston.

[chemnick]

I'd second that. If the valve doesn't want to reset you "might" be able to remove the rear line from the valve and persuade it to center. As for the brake light, make sure it has a good bulb and the wire is attached (obviously).

Chemnick

[Carmine]

Originally Posted by NEW64OWNER

Is the "brake" light on? If the valve moves the light should come on. Bleeding brakes will often move the valve off center due to unequal pressure in the brake line at the valve. Opening 1 front and 1 rear bleeder at the same time then pushing pedal a few times as if you were trying to bleed the brakes will usually re-center the valve as this creates equal pressure on either side of the valves sliding piston.

I don't know if the brake light is on or even works. I never had the switch on when I bled the brakes.
But, I can certainly power the system to see. Are there suppose to be wires that go to the proportioning valve?? If so, I don't have any. I guess I misunderstood how to reset the valve. I opened the passenger side front and rear bleeders, had the brake pedal press to the floor, held it there and tightened both bleeders. Then I bled the lines as usual. No changes.
Is it my understanding according to the above thread, that while the bleeders were open, I was suppose to push the pedal a few times, indicative of bleeding the brakes?? I didn't do that. I pushed the pedal once, held it down and closed the bleeders. I didn't think I was suppose to pump the pedal a few times with them open. Thought releasing the pedal with them open would draw air back into the lines. That's my fault. I can try it that way if that's what I'm suppose to do. Thank you, Carmine.

[78w72]

a life long mechanic friend of mine has always said to push the pedal very fast for the recentering trick, he said to stomp on the pedal like doing a panic stop2-3 times. supposed to help the valve move better than just light slow movement. worth a try.

[Carmine]

Originally Posted by chemnick

I'd second that. If the valve doesn't want to reset you "might" be able to remove the rear line from the valve and persuade it to center. As for the brake light, make sure it has a good bulb and the wire is attached (obviously).

Chemnick

If I have to go that far, how would I persuade it to center?? I don't have a clue. Thanks, Carmine.

[Carmine]

Originally Posted by 78w72

a life long mechanic friend of mine has always said to push the pedal very fast for the recentering trick, he said to stomp on the pedal like doing a panic stop2-3 times. supposed to help the valve move better than just light slow movement. worth a try.

Yes it is, thank you, Carmine.

[Carmine]

Originally Posted by 78w72

a life long mechanic friend of mine has always said to push the pedal very fast for the recentering trick, he said to stomp on the pedal like doing a panic stop2-3 times. supposed to help the valve move better than just light slow movement. worth a try.

Just tried that approach a few times. Those shoes just don't make any movement at all. No sound. Nothing.
What is Plan C?? I don't mind buying a new proportioning valve. Doesn't seem difficult to get to. My fear are the lines themselves. They are original and look like they might not take to removal from the valve, to easily. I see twisting and breaking in their future. I wanted to replace the brake lines, but not right now. Maybe it's best I buy them now and have them on standby in case things turn to crap, which they always do for me. Sorry about being redundant, but I should have walked away from this car, Carmine.

[Carmine]

Quick update. Could never figure out why the rear brake shoes didn't move. Ran out of patience and removed and replaced everything associated with the braking system except the shoes which were like new. Just finished installing a 7 piece pre-bent brake line kit I purchased at inline tube. This kit is great. Lines all bent perfectly. No kinks. No collapsing. Everything lined up nicely. Only had to slightly tweak one piece. I was never a believer in pre-bent stuff but I am now. I'm sure there are other vendors out there with maybe the same quality, but I highly recommend inline tube. A very good experience. I will be making a future purchase from them, of fuel lines for this car and brake lines for my '62 409. Going to bench bleed the mc and then install. Bleed the system and hopefully things will work properly, Carmine.

[Squidward]

I was going to suggest replacing the rear wheel cylinders...


They are cheap and easy. Yours might be rusted/stuck. Fluid would still flow, but you wouldn't get movement.

Have you had the rear drums off to inspect the hardware and star adjusters?

[Carmine]

The back brakes now work. Not sure how good because I haven't left the garage with the car yet. Only driveway testing. Hope I have enough inches of vacuum. Have the interior apart so highway testing will have to wait. The drum hardware all looks good, Carmine.

[Squidward]

That's progress any way you slice it. Good work, I hope the rest falls into place for you.