Brake pedal going to the floor
 

I have a 2000 F350 7.3 single rear wheel, cruse control, 56 thousand miles. I have since I bought this truck new in 2000 replaced three master brake cylinders. My brake pedal will all of a sudden go to the floor, I then replace the master cylinder and things are good again. This has happened four times since I bought the truck new. Now with the latest "brake pedal to the floor situation" I replaced the master cylinder AGAIN but this time it made NO difference..brake pedal still goes to the floor with the engine off or running....Any ideas what might be going on. Help Please

I would lean toward air in the system or a leak somewhere in a line or rear wheel cylinder.

Update....I took the new master cylinder back off and re-bled it the way I have always bench bled a master and NOT the way the instructions that came with the new master suggested. Reinstalled the new master and this time it worked. My breaks are working exactly the way they should again. BUT, the question still remains, WHY have I replaced the master four (4) time in the last 15 years of owning this truck. The truck does not get driven much "hence" the 56 thousand original miles. Is it possible that these aftermarket master cylinders are made of such poor quality?....Any ideas from anyone?