1979 Chevy El Camino proportioning valve

Front brakes don't work very well. Rear brakes are fine. New calipers, new hoses, new pads, new master cylinder, new rear shoes, springs and hoses, flushed lines with new fluid twice. Removed proportioning valve for cleaning, flushed each steel line, no vacuum leak, seems as though booster is working. When backing on loose gravel front brakes will not lock-up. On normal stop rear brakes seem to do all the stopping, as car does not dive downward on hard braking, car stays level. Is there a way to check these proportioning valves?