Whilst certainly no expert on brakes I experienced a similar problem recently. I had new ( genuine - purchased from patrolapart ) master cylinder fitted by a "reputably" brake business in Wodonga as mine was weeping. Asked them to adjust rear brakes ( drum ) at the same time. Picked up my ute and pedal was going to floor but would firm up with constant use . Took it back & they re bleed system , assured me that they had gotten all air out. Still no good. Went back , they said master cylinder must be faulty. Clamped off brake hoses as pearcey suggested - at least they knew about that technique. Pedal was solid. They then seemed a little lost and proposed a major investigation ( at my expense ) which I declined. Took my ute to another brake guy & discussed problem with him. He mentioned the issue with air being drawn in past the cups but he said it could just be a brake shoe adjustment problem. Left my ute there & he re bleed brakes just to be sure - got " bugger all " air out . Adjusted rear brakes shoes and road tested - all GOOD. My brakes are fine now, probably better then they have been for ages. Needless to say I won't be using the other mob again. Anyway just my experience.