My 49 Chevy Wagon has a 57 Chevy rear end with stock drum brakes. The fronts are GM M calipers and there is a GM combination vale in the circuit. I'm almost certain the plumbing is correct because I corrected an error right after I bought the car. The car stops well but in a panic stop, the rears lock up and that is a scary proposition. Today, I had a close call while out for a drive. I will investigate this more thoroughly this coming week but, assuming all is plumbed correctly and the combination valve is ok, what other things could cause this? Adjustment of the rears? Pushrod from the booster to master cylinder too long? Help me get the possibilities fresh in my mind.