1988 f350 diesel dually. Brake pedal goes 3/4 to floor then slowly "bleeds" down to floor. Have put in new, master cylindar, booster, pads, lines.. and no fluid leaking and still no brakes on back dually wheels. I am out of items to replace! and still getting no brakes in back and peddle slowly goes to floor.  

Where did you buy the booster? Possible bad part, or you have a vacuum leak from the manifold.  

Did you bleed the system?
You may have air in the lines.  

Does this rig  have rear antilock brakes if so the abs acumulator on fram rail can leak and cause symptoms like a bad master cyl. By pass with a short brake line and see if better if so its bad replace.  

good luck I've had the exact problem for 2 years. I've changed both calipers,wheelcylinders,2boosters, 3mastercylinders,vacume pump,and bypassed abs module on frame. I have no leeks like you either. I just started working on truck again and I will figure out, but in case you figure yours out first please let me know. I will let you know when I get a little more headway.  Never seen anything like it!!!  

I had a similar problem on a CJ-7.  Turned out the brand new power booster was bad, it had an internal leak.  Have the system pressure checked.  

did those trucks have a height valve in the back with the brake lines running to it like the toyotas have ive found those to fail and have a low brake pedal