if all 3 forward gears are OK... the problem most likely is the rear band / servo .
Check for engine braking in manual low gear. If the band is not holding in manual low , the engine will drop to an idle rather slow the vehicle down when you lift off the gas pedal
retrace your steps with the gaskets on the valve body and check balls for the valve body
Originally posted by 68chevelleSS454
Ok someone tell me where I went wrong. I rebuilt my trans because it was slipping in 3rd. Most of the seals in the trans were bad. Probably from heat because of no cooler. I put a cooler on it just after I got the car but evidently not in time. I used a B&M trans kit and used the street/strip specs. First, second, and third work great but NO reverse. The kit came with 3 gaskets for the valve body separator plate. Could NO reverse be caused by using a wrong gasket here? Anyone got any ideas?