also, how sure are you that the master cylinder is good?

are you replacing the lines because one of them ruptured and the brake pedal went to the floor? if so, that causes the mc to compress much further than usual. as a result, the seals inside slide against metal they have not rubbed against (and kept clean) in a long time. this can ruin the seals. (had it happen to me once.)

1994 4x4 22RE W56 truck