67 Corvette PB J56 Brake Problem

[72-SS-L48]

Hi Guys,

I just did breaks on a 67 J56 PB cars. The proportioning valve was rebuilt and the booster is brand new. We bled the front brakes and they are perfect. After pumping the pedal and holding it, the pedal sinks to the floor releasing the air when we opened the bleeder.

The problem is the rear brakes. We have fluid to them, but we cannot get pressure. After pumping on holding the pedal even when someone opens a bleeder in the rear the pedal remains hard and does not sink to the floor as it should, releasing the air.

We swapped a master cylinder, that is not the problem. We also changed the proportioning block, that did not change anything. We pulled the proportioning valve off the car and took it apart again, that looks to be ok.

What are we overlooking? Any help is appreciated.

Best,
Dave