helping a friend / brake pedal

[RBuhr69]

Thought I would reach out to you guys for some buddy help....I was over at friends house last night helping him bleed brakes after installing new master cylinder and power booster. Car is all all disc with proportioning valve, performed the bench bleed all good, bleed brakes as normal excellent flow to all calipers, no leaks what so ever...

the problem.....when you pump the brakes or apply brakes and have a good pedal, then let off for about 15-20 seconds then re-apply brakes the pedal goes to the floor........Then you can pump brakes back up, have good pedal let off then let sit 15-20 seconds then pedal goes to floor.

any thoughts what could be casuing this?