Power Disc Brakes not working on hard stop?

You have the 73 up A body booster. The 67-72 Kelsey Hayes 4 piston brakes used the dual diaphragm booster. In 73 they went to single piston caliper and the larger single diaphragm booster. i have several old brake application catalogs and the 67-72 used a different MC than the 73 and up. I don't know ****, but having the wrong booster could be part of the problem. 4 pistons push harder than 1 piston, hence the dual diaphragm booster on the 67-72 cars.