brake problems.. any suggestions???

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.  
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1970 Mustang, Supercharged

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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  

Is it possible to convert back brake (drums) to disc on 1995 E350 5.8, and if so .. any link to find conversion?