Final part of Full Cal of 4310 reverse drive wheels won't turn

[tennessee zim]

I am new to the forum but wanted to ask if anyone had done a full calibration on a JD 4310. I had to replace a reverse potentiometer so went ahead and did a cal on the forward and reverse potentiometers and then did the throttle and a full cal moving the fuse and all per the manual. It all worked perfectly till the very last step when I could not get the reverse to work ie forward, reverse potentiometer and throttle and forward drive circuit cal'ed but reverse would not move even though the tractor is up on blocks and I waited 25 seconds. I wondered whether anyone had come across this and what did they do

I have the checklist of electrical checks and then the dreaded pump failure diagnosis but thought if it was just the pump then forward wouldn't work either.....any thoughts?