Hello fellows,still having problems stoping truck.98 150,4wd,4wantilock.I have replaced rear sensor,master cylinder ,front flex lines,cleaned off front sensors ,adjusted rear brakes and bled the heck out of the system. I am not loosing any brake fluid. truck stops fine most of the time,other times pedal drops to floor and goes into antilock mode for no resone.Even when it stops good,you can still press pedal to floor after truck comes to full stop. Is the new master bad or is the brake booster going south on me?Don't know what to do next!

 

Check your rubber brake lines for dry rot or cracks.

 

sounds like a bad MC... there is probably a problem with one of the seals.

There is also a possibility it is a contamination problem - if a particle becomes lodged under the seal it can cause a bypass condition which would give you the intermittent pedal to the floor condition.
BUT - you said you have bled the brakes a lot so this probably isn't the case, assuming the fluid you put in is clean. (Did you recirculate the fluid during bleeding? If so, watch for any small particles in the fluid.)

If you just replaced the MC, I'd take it back and have them exchange it.