Hello guys, new to the forum here. Been lurking for a couple of days now trying to figure out the brake problem on my 02 f-550 7.3 wrecker truck. The pedal started going all the way to the floor one morning right after I cranked it. So I changed the master cylinder and it was still doing it so today I changed the hydrobooster ? I think it's the name and I figured out that the problem is that the rear wheels DO NOT lock up. Only the front. Any tips and ideas will be greatly appreciated! Thanks in advance.

 

Disc rear brakes, factory axle, ABS? Some outfitters change stuff.

The booster wouldn't cause this. My first guess, other than a leak which should be easy to see, is that the master had gone bad and now after replacing it you have air in the rear system. If a wheel bearing has gone bad allowing the rotor to wobble around it'll also give a low pedal. If it has a weight sensing proportioning valve (that might not be the right name for it) on the rear something may have gone wrong with it killing the rear brake pressure, but I don't think it would cause a low pedal.

 

It has disc rear brakes but no bearing failure, everything is in place back there. It does have abs. Factory everything

 

Was the brake system power-bled after changing the master cylinder? Pump and squirt does not always get the air out of the ABS system.