New Brake Master Cylinder Needed??, diagnosis inside Options

[BonzaiCelica]

I replaced my brake master cylinder with a new Raybesto unit as well as a new proportioning valve (OEM). My problem is I will get air pockets 5 min into driving, pedal sinks momentarily and then brake feel is back to normal

While bleeding brakes, once i crack a caliper open, air bubbles do not come out till about 10 seconds later.

-Booster Operating check OK
-Booster Air Tightness check – engine off OK
-Booster Air Tightness underload OK
-No Leaks at Banjo Bolts
-Soft Lines OK
-No rust between shims and pads
-Caliper Pistons Seals not torn or leaking
-Caliper Pistons move freely when pressed inward with bleeder valve open