Dale,
The spool valve on my full tree brakes are spring loaded and I think that the ones on a pro tree brake are also. Was the transmission working before or did you just get this one? If you just got this one, I would unscrew the transbrake solenoid and make sure that the spool valve is there. You will need to drain some fluid or have the rear of the transmission higher than the front. Maybe somebody went to change the solenoid and the spool valve came out and didn't get replaced.
Bill Mitchell