Dent, I have the same problem on my flip truck. I've replaced the rear abs module under the driver floor on the frame. No change.... I bought the F450 master and hogged out the holes so it would bolt to the booster. No change. Pedal still falls. All the brake hardware is fine. All pads and shoes about 1/2 to 3/4. The only thing I haven't replaced is the booster. This problem is driving me crazy!!! I've missed selling the truck because the potential buyer didn't like the brakes. When you apply the brakes, it's windshield city!!! I mean this truck litterally locks down. If you apply them really lightly, they work fine. But if you are holding the brakes going down a steep hill, the brakes fade and the pedal goes down. That same buyer called me back and said it couldn't be fixed because his mechanic told him so.........That alone is the reason I've not advertized this truck. I can fix anything.... And I want to fix this just to prove him wrong. I will replace the booster as soon as I have the chance and let you know. And I will be watching this thread closely.