the pedal in my RX is soft and it won't build pressure, I have replaced the rotors,pads, calipers, master cylinder(all of these came off of a 89RX the rotors/pads have about 500-1000 miles on them and the calipers look good (better than the ones that came off)) I bleed them in the X pattern (driver front,pass rear,pass front, driver rear) three times, once with the engine off, twice with the engine on. I bleed new brake fluid thru each bleeder screw, no air came out the last time I bleed them. what am I missing? could this be air in the master cylinder? I also checked to see if the lines expaned when somebody pressed the pedal, they didn't. thanks for any help you can offer

Park it and check for leaks.

The air could be in the master cylinder, especially if you didnt bench bleed it before putting it in.