DIFFICULTY WITH BRAKES?

i have an old challenger with power brakes, fnt disk (from a 73 dart) and rear drum (which both are not the problem). i have bleed my crakes numerous times checking the unions for leaks, but nothing. i can pump up the pedal to stiffen but then it looses pressure all parts are new, and i've replaced the master cylinder more than once. im lost, have no idea what it could be. please help