Changed Master Cylinder twice, still getting air in the front brakes.

CK RUBBER HOSES FOR CRACKS

Had same problem on a Corvette, found caliper piston seals would not leak fluid 'out', pressure would seal it off.... but air was getting 'in' by the piston seal... Just another possibility... Rare but possible!... By clamping the rubber flex hose, one then the other, and testing brake pedal each time. You should find the problem area!!... Will be a big difference in pedal travel and feel...