2004 F250 Lariat 5.4L
New Front calipers, new Master cylinder, new Booster, bleed each wheel 20 times, no leaks anywhere,
still at a red light the pedal sinks about 3/4 to the floor at that level the brakes finally hold.
any help on this would be appreciated.

 

If you don't run the engine, apply the brake pedal a few times to use any vacuum left in the booster. Then when you depress the brake pedal and hold it there , if it sinks down, master cyl is defective..
It is difficult to determine just how much pedal pressure will stop a stationary vehicle , the pedal may feel soft, but could stop the vehicle way before the pedal gets to the same position as when stationary.
From your description it sounds like a bad master cyl. For some reason have come across a few bad ones lately, don't know why , just return it and get another , but don't forget to bench bleed it before installation and try not to let any fluid run out before connecting lines.

 

hopefully by now you got it fixed-- if the has drum in rear - I would look at the proportioning valve as a potential problem--
lot of people don't understand that this can cause a problem with the rear brakes