1973nomega

I installed new slim brake booster and master cyl. And both front calipers bout a year ago. Car has sat now I've tried to bleed the brakes, starting with pass rear, drivers rear and so fourth. Both backs bleed fluid and air bubbles I've spent at least two hours bleeding them. The fronts will not bleed at all no pressure with vacuum pump. All the fittings, bleeders and hoses are tight. I've tried the old fashion way off bleeding with someone pumping and holding the pedal and gravity feed but the fronts still won't bleed. HELP!!!!! Please

rustyroger

If you have spent two hours bleeding the back brakes and haven't cleared air from them I think you have a bad master cylinder. It sounds like the rear seal is letting air in. If this is the case you will never get the brakes bled. It could also account for no fluid from the fronts.
It seems odd that you get nothing at all from the fronts, however to check this out trace the brake lines back to the master cylinder and starting from the master cylinder crack off the fittings and see if fluid comes out. when you get to a fitting where nothing happens you have found the problem.


I'm assuming you are keeping the brake fluid topped up as you bleed and you have checked for leaks?. Sorry, but sometimes we miss the obvious.


Roger.

MDchanic

1. Did you bench bleed the master cylinder in a level position first?

2. Are you depressing the hold-off valve button while you bleed (if so equipped), as per the instructions in the Chassis Service Maniual?

- Eric

joe_padavano

I have to say that I've never depressed the button while bleeding brakes and never had a problem.

To the O.P.: Once again, this is common sense troubleshooting 101. If the M/C is bled, then work down through the front brake plumbing. Sequentially loosen fittings and see if you are getting flow there. At some point, you won't, which is where the blockage is located.

rustyroger

When I said sometimes we miss the obvious I must emphasise that includes me.


Roger.

MDchanic

I think I bled my brakes a couple of times as a teenager without depressing the button before I read the instructions in the Chassis Service Manual, and I didn't have a problem either, but, hey, who knows, right?

- Eric

joe_padavano

These days I use a pneumatic vacuum bleeder (the best tool I've ever bought) and I don't even need to bench bleed the M/C anymore. This bleeder just sucks the air out in record time. One pass at each wheel and I'm done.

4speed455

Make sure the left caliper is on the left side and the right one on the right side. If they are reversed the bleeders will be on the bottom and you never get the air bubble out of the top of the caliper. Ask me how I know. Lol

joe_padavano

While reversed calipers will prevent you from bleeding all the air out of the system, even with the calipers reversed you'll still get fluid out of the bleeders. The O.P. says that nothing is coming out.

1973nomega

I removed a rubber cap from the front of the proportioning valve and stuck a small nail in it, it seemed like it was pushing another pin? I had someone pump and push the pedal to the floor fast and tried slow with one rear bleeder cracked open then shut then open etc. still nothing. I put the two clear hoses and plastic fitting onto/ into the master cyl to bench bleed again it had very tiny bubbles in the hoses I could not get them to clear.

MDchanic

You are holding the master cylinder level, right?

- Eric

1973nomega

I keep the fluid in the master cylinder topped off the whole time. I'm thinking the combination valve might be bad or stuck? I removed the pin from the top where the single wire is attached plus removed the rubber cap on the one end and I'm not sure what I'm looking for from there? I've read a pin is supposed to be centered but I don't see a pin?

1973nomega

Update today I bought a reman master cyl. And new proportioning valve bleed the master cyl. In a vise. I put thread tape on all the fittings. All 4 corners now bleed but still have some air in them. When I push the pedal to the floor( which has no resistance) the rear of the master cyl. Bubbles and shoots out like it's spitting at me( maybe I angered the brake gods..lol) the front chamber is just fine but still no brake. I'm at my wits end with these brakes.

boese1978

It is common that when using the "vacum bleeder" clear hoses that come with the pump etc to assist in bleeding they will draw small amounts of air in at the bleeder where the hose slips on it.

rustyroger

See if you can bleed air from the brake line fittings to the master cylinder and work backwards to the bleed nipples on the brakes. I have done this when I have had a system that refused to bleed in the normal way.
I know hydraulic brakes are manufactured products that respond to the laws of physics, but occasionally they will behave as if an evil demon has taken charge.


Roger.

1973nomega

Here's a question if the rod that goes through the boost is sticking out to far would that over extend( pushing it in too far) the master cylinder and cause the seal internally to leak? Does anybody know how far it should stick out with the pedal pressed down?

MDchanic

I don't know offhand, but there is a specification for brake pedal pushrod length or play in the Chsssis Service Manual.

- Eric

joe_padavano

Yes, that is a possibility, but have you changed anything about the booster? If not, then it is unlikely to be the problem. One thing you can do is disconnect the booster pushrod from the pedal and verify that there is no preload in that rod with the pedal all the way up. If there is, it is easy to adjust the clevis on the pushrod to eliminate preload. There is no adjustment on the pushrod between the booster and the M/C, which is why it is unlikely to be the problem.

1973nomega

So I adjusted the pushrod( it had a small amount of adjustment) and I spaced the master cyl to 1/8" and now the front chamber spews and I can bleed the fronts and passenger rear just fine but the drivers rear will not get all the air out I've filled it like 6 times never letting it go empty but still no brake pedal pressure?

Bernhard

You should never use tape on brake fittings its the flare and seat that make the seal.
The brake rod adjustment is critical and must be with in factory specs.I worked on a car were someone took all the play out of the rod and the brakes were always on lightly/ dragging they could not figure out why they used up there brakes so fast.To loose is no good ether as it can effect full brake travel that's why it has to be in spec. If the brakes worked fine before your upgrades that's were I would start looking for trouble first.
I have personally never had good luck with remand master cylinders they never worked as good as a new unit. I always found that I could never get as good pedal pressure and they always failed with in a year of being installed.

joe_padavano

I would be worried about doing this. There should be no spacers between the M/C and the booster. Again, the pushrod between the M/C and booster has no adjustment and if it worked before, there should be no need for any spacer. More importantly, the M/C must clamp against the rubber boot at the face of the booster to properly seal the booster. Any spacer will prevent a proper seal and eventually lead to a leak in the booster.