Ok, When I reverse and then put my truck in drive my brake light goes on and my brake pedal goes down to the floor. it is fine after i pump the pedal and the light goes off. the pedal has a great feel after that. but as soon as I reverse it happens again. I already had a mechanic check it and he had me replace the Master cylinder and rear brake shoes. But it still does the same, I think my mechanic ( he is old school , used to work on pre 1972 trucks) might not know that much about bleeding the abs system. would this be causing it. please let me know