I have a 700r4 automatic tranny that was just overhauled and the oil pump replaced. backed the car out of the garage and it ran just fine. let engine run, as to check the tranny fluid, and filled to correct level. tried to take car out for test drive and reverse would not engage. it worked to back the car out of the garage but after warming up engine and filling tranny fluid the car lost reverse gear. all other gears work just fine and operate correctly only reverse will not engage. what could be the main problem? thanks in advance to anyone that can help.