I have a problem that my break pedal goes soft after five minutes of driving. A quick dobbel pump of the pedal and it's hard and feels right again. Suspected trapped air in the brake system but after traditional bleeding, bleeding with vacuum and reverse bleeding (pushing fluid from calipers) I find it hard to believe that there is still air trapped in the system.

Current brake system is based on SSBC A-156 four piston calipers with a 11.25" rotor, OEM drum drum distribution block, SSBC A0707-1 prop valve to adjust pressure to the rear circuit, Doctor Diff 15/16" master cylinder (manual brakes) and Doctor Diff 10,7" rear disc brake kit based on Ford Mustang cobra OEM calipers.

I had the same problem last summer with the same brake set up except from 10" drums in the rear and SSBC 1.032" master cylinder up front.

Starting to lean towards that the problem could be related brake pad knock back so I checked for play at the wheel bearings but they nice and tight.

Anyone have any Idea that could cause this problem?