Does your mower use Hydro-gear transmission. if not sure look on top of transmission & you will see manufacturer & Part code.
My Victa Pro16 won't drive in either direction. Plenty of oil, drive brake is functioning ok, Actuate/Bypass lever is Actuated & if I jack up rear wheels they respond to pedal, but very little torque. I am going to drop the transmission & check for obvious problems with the integrated pump.
I suspect that this is same problem you have.
cheers..B
