I have a 67 Malibu, which was originally a power drum brake car. I have converted the front brakes over to disc. I am using the original booster, along with new parts (front disc master cylinder, calipers, and combination valve) for a 71 Chevelle. The entire brake system has been replaced front to rear. The brake pedal is very firm, but it does not require a great deal of force to stop the car. The bigger problem is the rear brakes lock on hard stop. Is there something that can be done with bleeding the system or resetting the combination valve?

Same thing happens on my car think it has to do with weight transfer....?

The rear brakes sound like they have too much bias. Might have to install an adjustable prop valve as it can be hazardous to your health when the rears lock up,first.

Surely someone out there has used this combination of parts to convert a 67 to power front discs? All the parts except for the stock rear drums and the stock power booster are new parts from a 71 Chevelle front power disc system. I'm inclined to believe something is wrong in the setup rather than having the wrong parts. Could be a faulty combo valve, although it is brand new. Is there a particular procedure for bleeding a system with a combo valve? I used the old standard method, first gravity bleeding, then pressure bleeding, starting with the rear first. Could the combo valve need to be reset?