Brakes need pumping

I have a 72 swinger with power brakes and BBP discs in front. If the car is sitting for a while the brakes need to be pumped once or twice and then will work fine. Most of the time driving brakes work fine. Every once in a while brakes will be weak, give them one pump and work fine. I thought it was air because I could not get one front bleeder open. After alot of Kroil and heat I finally got it open and bled. Very little to none air. The brakes are firmer but still occasionally need a pump. What do I look for: air in system, bad master cylinder, bad check valve for brake booster? Engine is stock worn out 360, still has 18" at idle, did not check when in gear. Just wondering where to go next?