Brake problems: no rear brakes

[twsutt]

I've just replaced my Brake Booster and Master Cylinder. The push rod is adjusted to 9mm past the aluminum (not aluminium) spacer. I bench bled the Master Cylinder with with a vacuum bleeder (mityvac) but when I try to bleed the rears I get nothing just an air tight vacuum - according to the pressure gauge on the mityvac and the sound when the hose is removed from the bleeder screw. (Yes, I loosened the newly teflon wrapped bleeder screws.)

So I tried using the tube into a jar while pressing on the brake pedal - nothing. I placed the jar and hose on the Master Cylinder rear bleeder screw (furthest from the firewall) and pressed the brake pedal - nothing again. I placed the jar and hose on the front bleeder screw (closest to the firewall) and pressed the pedal - brake fluid smoothly moved bubble-less through the hose into the jar.

The engine was running and the pedal went all the way to the stop (not technically the floor) with the same resistance whether any of the bleeder screws were open or not and the "brake" light on the speedo would light at the pedal's full extension.

When the engine is off the brake pedal's resistance is the same and the pressure can't be "pumped up."

Then brakes worked before, but the Brake Booster was hissing and the braking was very soft - I couldn't lock the brakes up on smooth pavement.

Did I get a bad Master Cylinder? Do I have the push rod misadjusted? I don't think that it's a slipped reaction disk, because the front brakes can be bleed at both the wheels and the Master Cylinder. Anyone have any Idea what my problem is - other that that my brake don't work? (That is not an invitation for a smart-a$$ remark.):tapemouth

Any help is appreciated. Thanks.

~Terry

[Walter Moore]

Do the front brakes actually work? (I know that you can bleed them, but do the calipers grip the rotors?)

It sounds like there is kinked steel line somewhere in the rear circuit, but it could be multiple problems. Did you change any of the wheel cylinders, or brake hoses?

Initially I had problems with a soft brake pedal and no rear brakes. In my case it turned out to be caused by the "New Old Stock" braided steel brake hoses, which had gone bad with age. I replaced them with real new stock Bendex hoses and my problems went away.

Somehow I doubt that is your problem. On the other hand, I did once have a new brake hose that wasn't drilled all the way through on one of the fittings. No amount of vacuum would pull fluid through that hose.:surprised

[twsutt]

The brakes work - barely. I've pulled out of the garage and back into the garage (about 2-3 mph) and the brakes were enough to stop the car. I had my hand on the parking brake just in case I couldn't stop. No hoses or wheel cylinders were changed.

I think the part that concerns me most is that no fluid comes out of the rear bleeder screw (furthest fom the firewall - marked "R") on the master cylinder when the pedal is pressed.

[m4xwellmurd3r]

Hmm...you said that no fluid comes out of the rear bleeder screw on the master cylinder, but it does in the front right? That REALLY sounds like the master cylinder is dead. I didn't use any bleeder machine though, I installed it into my car, and bench bleed it using the brake like normal, then hooked up the lines and I actually didn't have to really bleed it after that. I did a pretty good job of not loosing any fluid so the brakes are nice and firm now.

That's really strange that nothing comes out though..

[Curtis240Z]

Try completely remove the rear master cylinder bleeder screw and press the brake pedal. If fluid comes out you have a clogged bleeder screw, if nothing comes out you have a bad master cylinder.

[twsutt]

The rear master cylinder bleeder screw isn't clogged - fluid comes through when suction is applied. No fluid comes through the screw when the pedal is pressed. I didn't know if this was a pushrod length problem or bad master cylinder (or the dreaded reacton disk issue.)

[m4xwellmurd3r]

Well, if the front brakes are working, then it means the push rod is applying pressure to the brakes. iirc, the front section are the rear brakes, and the rear section are for the front brakes (it's like that on my 280 at least)

[xray]

.....

[sblake01]

Sounds like you got a bad master cylinder. If your're getting front brakes, the push rod is moving both pistons in the master cylinder. The problem must be with the secondary piston (rear brake; farthest forward) either the seal or the return spring. Disregard the wired caps, this scan if for a 78:

[twsutt]

If the secondary piston (rear brakes) of the master cylindar was shot would that keep me from being able to "pump up" pressure in the brake system when the engine is off?

I guess the easiest way to tell would be to exchange the MC for another.

-Terry

[m4xwellmurd3r]

pretty much. If the front brakes are fine, and the front piston in the master cylinder is shot, then the fluid will bleed internally and it'll never build pressure. I know when the rear piston went out (front brakes) if i jammed on the brakes it was solid, but then the pedal would sink to the floor as the fluid bleed past the seals.

[twsutt]

Thanks everyone. I'll remove the MC this evening and hopefully the parts store can have another MC for me by the weekend.

I'll post the results once a new (rebuilt) MC is installed.