ABS (Anti-lock Braking System),Rear brakes not grabbing

I am having trouble with my rear brakes not grabbing very well. I set up the rear on jack stands and could hardly stop the rear wheels with the brake pedal. My front brakes are in good condition and lock up in gravel, but not the rears. I bled the rear brakes with a pressure bleeder and the fluid came out with no bubbles. The brake pedal is firm. The master cylinder looks like the original. Can the master cylinder fail for the rear brakes? Could it be the proportioning valve built into the Kelsey Hayes 4wal ABS system? Could the proportioning valve have an isolation circuit that is stu