TF-727 Reverse Trouble

If you remove the valve body (isn't hard, unscrew the larger hex head bolts), you can test the servo by applying air pressure to one of the holes in the casting. The FSM and the tranny manual should detail the procedure. You should hear the servo piston move and see the rear band tighten. Check all the pistons while you are doing that.

However, hard to tell how tight it pulls, so either way you probably need to pull out the piston to make sure it hasn't cocked. That is an endemic problem and there is a better after-market aluminum piston improvement that costs <$20. You need snap-ring pliers to remove the piston. I don't remember if you must remove the valve body to get at it.