'71 Brake bleed

[Jeremy Hedges]

I have a '71 that I can't get a hard pedal with, it has power brakes, I have replaced the rear two calipers, a new master cylinder, all 4 rubber hoses, I am getting a full stram of fluid to all four calipers. The front brakes seem to work well, when I have the car in the air and put it in drive I have no rear brakes, I am using a vacuum pump to pull the fluid thru the rear caliper bleeders and I am getting a full stream on both bleeders per caliper. I can nver get a hard pedal either with the car running or not. The rear steel lines seam to pull fluid just fine, I just can't get any pressure to the rear calipers thus I never have gotten a hard pedal. I also have adjusted the rod going into to the master cylinder and it makes no difference. I have a '65 with PB also and I had no problem at all bleeding the brakes(it was a complete restoration though, and the '71 is not)
Any help would be appreciated.

thanks

[Patrick H.]

Re: '71 Brake bleed

Jeremy,

Seriously consider either A) using a friend to push on the pedal while you release the bleeder screw, for a positive-pressure effect, or B) renting or purchasing a pressure bleeder.

On my Dad's 72, my 71 and my 70 Cutlass, I always have better luck getting a hard pedal if I have an assistant pushing on the pedal (3 pumps between bleeder releases) than if I use a vacuum bleeder. I use the vacuum ONLY to get fresh fluid to the caliper, then use the assistant to finalize the procedure.

This is assuming your new MC is properly bench bled AND is not still causing the problem. I had a brand new MC that was machined incorrectly and gave me fits tracking down the problem last summer on the Cutlass.

Patrick

[John L.]

Re: '71 Brake bleed

Jeremy,

I recently experienced the same problem on two different cars both 72's. Soft pedal and low pressure to the rear calipers.

The first time was the proportioning valve had moved because it sensed low pressure to the rear calipers. The brake light came on each time I tried to bleed the rear calipers. I had flow but no pressure. To get the valve to recenter I loosened a front bleeder and hit the brake pedal as hard and fast as I could with my foot. On the second try the valve reset and the problem was solved.

The next time I had the same problem on a different car. The valve would not reset for it was stuck. I replaced the proportioning valve and it corrected the problem.

Hope this helps.