power brake problem

[RShaips]

My brakes don't seem to work correctly, when I mash on the pedal the truck stops but it takes longer than it should. I have a 67 SB on a 71 frame with disc brakes up front, the power booster came off of a 72 4 x 4 truck, do I need to put a different portioning unit on it ,since it came off a 4 x 4 ? The brakes feel like they have air in them, but I have bled them and also have installed new wheel cylinders. Thanks for the help