Just installed a factory PB setup on my '70. Rebuilt an original master cylinder and booster and put them on the car.

Bench bleed the master cylinder and went to bleed the brakes and I got nothing from any of the calipers. Nothing!

Blew air thru the brake lines were they go into the master cylinder and fluid comes out of the caliper bleeders.

So where's the problem?

Does a PB car need to be running (applying vacuum to the booster) during bleeding operations?

Did I do something wrong when I rebuilt the master cylinder like getting the secondary stop screw set in the wrong spot, or installing something else incorrectly within the master cylinder? I took the master cylinder back off and used a screw driver to activate the piston. Fluid squirted out of each port.

I also checked the piston travel distance with the travel distance of the booster rod and it appears as if they both travel a total of approximately 1".

I'm leaning toward something inside the master cylinder. I just ordered another rebuild kit but before I rip her apart I thought maybe I'd ask for your thoughts. I want to keep the original master cylinder which is why I'm going the rebuild route.

Thanks for any thoughts you might offer.

 

I just used a new M/C and new ss calipers on my son's 75. Couple of things ma help you.

First, I used a new M/C vs rebuilding the old M/C so be sure you have the piston in the correct order,although I'm not sure you would be able to re-assemble it otherwise. How does the bore look?like calipers any pitting will cause a problem. Also the PB M/C has a larger bore then a manual M/C, I think it's 1.125" for the PB but you should double check.

Bench bleed it. I found the red plastic bleeder kits you find in auto parts store are junk and the threads are not metric like I found on my 75 M/C.. This caused air to be sucked in the M/C. I then tried using the motorcycle type of bleeder-a syringe to push the fluid into the M/C. this worked ok but I ended up going to NAPA and gettignthe metric conversion brake lines and fitting them to the M/C ports and running them inside the M/C -below the fluid level. This worked the best. Yes there is about 1" travel.

Once I got the M/C done and installed I used a Motive pressure bleeder. I had one rear bleeder that wasn't pushing fluid out-no matter what I did. Ifinally swapped it out for the original bleeder and found the hole was undersize and not drilled through. Once the old bleeder was on I was able to bleed the system and it's been great since. I had already rebult the rear bearings and had the rotors within .001-.003" TRO so this will also affect your situation.

Good Luck, I would start from the beginning to see if you might have over-looked something.
Gary

 

I'm certain the caliper bleeders and the brake lines are good, as I gravity flushed them with denatured alcohol about 2 years ago when I replaced all my calipers. At that time all bleed perfectly. Haven't put more that 100 miles on the car since then.

What about having the motor running to create vacuum in the booster during bleeding? Is it required?

Also, I didn't replace the tube seats/check valves/springs in the master cylinder outlet ports, just the primary & secondary cylinders and related seals. Could bad check valve parts cause the problem I'm having? I'm doubting these check valve parts are the problem because bench bleeding worked ok.

 

I had a similar problem. Bench bled the master and it appeared to be working, but when bleeding no fluid at calipers. I took off the brake lines at the master and tried pumping with the pedle and no fluid came out. I bought another NEW master cylinder and that cured the problem. It is possible that the master cylinder push rod is not long enough. If I remember they are adjustable. It is possible to have push rod too long and in that case the brakes will lock up. According to the Chevrolet Chassis and Service Manual DO NOT use the power assist while bleeding the brakes.

 

Push rod travel/length was my first thought. That's why I checked the piston travel in the master cylinder against the total travel of the booster push rod. They're almost identical (within 1/8"). I believe the only travel adjustment is at the clevis were it attaches to the brake pedal itself. There's not a whole lot of adjustment here though, just enough to regulate free play of the pedal and to coordinate with brake light switch operation.

I used the plastic bench bleeders on the m/c while it was on the car and used the brake pedal to in effect "bench bleed" the m/c while mounted. Worked fine, just as it did on the bench. Pumped fluid back into the reservoirs with no problem. Couldn't tell if it was "back sucking" fluid though. That's were my concern with the check valve parts comes into play.

 

My master cylinder bled fine also, but when I disconected the brake lines at the master with the master on the car and pumped the brake pedal no fluid came out of the ports on the master cyliner