Thanks for the reply, don't like your answer but I think that you are right on. It also might answer an ongoing prob that I've had with my f250 superduty diesel. It has rear ABS and a few years ago when I had brake probs it developed another problem. It would NOT relieve the brake pressure in the rear brakes. After about the 3rd time you use the brakes I have to crack the rear bleeder because the pressure builds and rear pads brings truck to a standstill. The fluid is not returning to the m/c. (I assume its not returning through the ABS system)