HELP!! 93 F 250 Brakes

Not to long ago I totally redid the brakes on my truck, new master cylinder, calipers, and brake cylinders. And I’ve been having a problem with what I think is my master cylinder. I’ve bleed it out probably 10 times, no air in system. Pump brakes up letting the pedal move all the way up before hitting them again. Brakes will stiffen after about 10 pumps with no fade. Let pedal go give it about 10 seconds the pedal will go right back down to the floor. I can feel the back brakes engaging when it gets really low. I was looking at a parts web site and noticed that for the 95 if it has low pedal to go to a 96 cylinder. Has any one got any ideas or has heard of this before?

did you bench bleed the master cyl before you installed it?
try jacking up the rear and spinning the tire as someone pushes the brake pedal,the rears might need to be adjusted out some more.i usually adjust them out till i can barely hear them touching.

Either the rear brakes are WAY out of adjustment (you did make SURE to get the adjusters back on the correct sides right, they are different), or the RWABS module is bad. Given your specific symptoms, my money is on adjustment.

Have you checked the power brake booster?? Alot of times if the booster is bad it will fill spongee..Does the pedal go's to the floor when the truck is off or with the engine running?If you it go's to the floor only when its running most likely its the booster or the line going to the power stering pump if you have vacume assest sterring that runs off the booster.Seams to me like you checked everything else, well thats my 2 cent$...

Sounds like adjustments as said above, Did you replace the whole rear drum ass. ? I've found that brake cylinders go bad sometimes.. could be a problem. (Had to replace mine over a simular problem)