sound like a regulator
needs to be adjusted
to allow more fluid to flow faster
i would follow line off pump or check around pump im not sure on this peticular model
I'll try to help with this one.
I would suspect that you have a resolver ball leaking in the pilot joystick. You should get pilot pressure to the pump swashplate from the joystick when the lever is moved to the forward position.
It is port D on the joystick and port X4 on the front pump. The port is on the pump closest to the engine on the left side. It will be a small line going into the swashplate servo or actuator.
For the motor to lock up only in the forward direction and not the reverse direction, I would say it is in the joystick since the swashplate will move in both directions and the track does not turn without a command like if the swashplate actuator was getting stuck or the seals were blown.
Since it works in reverse and for the other track in both directions, pilot pressure is ok in the system. If the brake in the motor were locking up the direction shouldn't matter and both motors should do it because the parking brake oil splits after the solenoid and goes to both motors.
The swashplate is spring centered and when pilot pressure is low enough, it will center up and lock the motor hydrostatically.
That is the left side when looking towards the pumps from the front of the machine, not in the seat. Here look at this view of the pump.
This isn't the best picture but it is all I can find.
Just locate the hose labeled 5 back at the joystick...or you could swap at the pump. The forward ports are on the same side. So you could swap lines there if needed