RXSETH

My transmission won't go in reverse with the car on or off. I have tried pumping the pedal a bunch of times and it still won't engage. All the other gears shift fine while driving and while sitting still. The transmission was fine the other day and there were no noises or anything that would indicate something breaking. I tore the shield off the bottom and drained the fluid. There were no abnormal metal shavings and gears 1-4 looked fine. I have yet to tear the transmission down further and was wondering if anyone had any advice for this problem.