Brakes won't hold pressure

i just had a problem similar to this last week. while my motor and trans are out i replaced all the brake lines and plumbed in a adj prop valve for a future disc brake swap. i bled the system like you and got good pedal pressure. after i let off it and pressed again it went to the floor. after a couple of times of bleeding the system with no improvement. so i made sure the prop valve was adjusted so the rear was getting less pressure. and had someone pump the brakes so i could look to see what was going on. i found the problem in a few minutes. as the brake were getting pumped i noticed it looked wet on a line coming out of the master cylinder. nut was tight but just not enough so it was sucking air. went 2 flats more, did a quick bleed and problem fixed. might be a good idea to get a helper so you can see whats going on. good luck