Power Brake Issue

[R6RaVEN37]

Ok everyone, I need some help before I lose my sh*#t.

I just did a front disc conversion on my 69 Coronet and I am having a weird issue with the pedal. A little background before I get to the issue. Master cylinder and booster (bendix style dual diaphragm) are brand new. Master was bench bled. A new distribution/proportioning valve was installed. New vacuum hose installed (vacuum is god at 40). Brakes were power bled twice, both times I put about a quart of fluid through the system.

So now the problem I am having. After bleeding the system I have a good pedal. As soon as I start the car and push on the brakes the pedal goes to the floor and stays. If I pull the pedal up and push it again the same thing happens. If I disconnect the vacuum hose at the check valve and then work the pedal a few times I get a good pedal. As soon as I hook the the vacuum hose back up and push the pedal it goes to the floor and stays again. If I turn the car off and work the pedal a few times I will get a good pedal again. Could the booster be bad?

Any help with this would be appreciated. I am at a loss and this is driving me crazy.

 

[MIKESPOLARA]

Bad booster

 

[R6RaVEN37]

Bad booster

Well that just sucks since it was brand new. Guess I will be calling the place I got it from for a replacement.

 

[Roger63]

I have a brand new one if you have any trouble

 

[steve340]

Have you got some clearance on the booster rod and the brake lever?
There should be a slight amount of free play to allow an air valve at the back of the booster to operate.
Not to be confused with the clearance in the engine bay of the master cylinder/booster pushrod.

 

[Ron H]

Went thru a friggin nightmare with my conversion (also manual to power brakes) doing a shitload of things to solve it; but I could still achieve 'some' braking action and suffering pedal-sink. I did all the booster checks that checked ok. I installed a reserve vac cylinder, I re-bled the MC though finding I couldn’t fully get a bubble-free flow on one reservoir. Checked the gap btw the booster and MC, getting a gauge for this. Re-machined the pedal linkage as the one in the kit was a POS and futzing with the pedal ratio. Checked to ensure the calipers were installed correctly (bleeders on top). Also, though found the pads were binding on the caliper guides with evidence of a little damage on the guides. Deburred the pads and the guides, greased those…and replaced the MC and the booster. Also made sure the rear drums were adjusted properly and seeing...well the drums were pretty worn, replaced them. Problem solved…what did it? Not sure! Friggin works great now

 

[Nate S]

Those new dual 8” boosters are junk. The most common problem is the pedal sticking down when hot but pick your issue after that. I’m on #3 (warranty replacements) and this one sorta sucks less. When I get around to it I’m putting a factory booster in.

 

[Dave6T4]

I have used dual diaphragm boosters and MC from late '80's - early '90's B-vans. The assembly is mounted just under the hood on a 90 degree bracket. It is a lot more accessible than getting one from a car. No climbing under the dash. You will likely have to modify mounting holes on b-body firewall.

 

[ckessel]

Also make sure the booster hose you are using is rated for PB. It should be 11/32" and say for use on PB. Correct PB hose has extra reinforcement in the liner to keep it from collapsing under vacuum unlike 3/8" fuel hose.

 

[R6RaVEN37]

So I took the booster out and reinstalled the aftermarket single diaphragm booster that was on the car before I started the swap. I have good pedal now, but the brake pedal still does not return. Is this an issue with the master cylinder?

 

[Roger63]

The booster has a spring inside and that is what returns the pedal

 

[steve340]

If the pedal goes right to the floor I would say yes the master could be faulty - but keep in mind you could have more than one component/thing faulty.
Assuming the booster is the problem - once the hydraulic pressure is at maximum the master cylinder can produce in the front and rear circuits and if all the components are capable of holding that pressure. The booster alone should not be able to force the pedal to the floor.

 

[Ron H]

Check the rod gap tolerance btw the booster and MC? Also as was posted, there needs to be slight pedal free-play at the home position to ensure the booster is fully released. Next could be measuring the pedal ratio. Yeah...there's a next; but check this out if ya haven't.

 

[Fran Blacker]

Bendix booster in SM.

 

[1STMP]

Isolate the circuits. Pinch off the hose for the
rear brakes. If the problem persists, pinch off
the front. If there's still a problem, the culprit
is likely the MC or the booster or air. I had a similar
problem and ended up using a piece of wood
the same thickness as the front rotors, removed
the calipers, and rotated the caliper bleeders
where they were truly at their highest point.
That did the trick. My pedal is excellent.