Another thought I have seen a couple of times Is the lever that is attached to the shaft going into the travel pump is splined to the shaft and may be stripped. you might check that before you dig into the motors. As far as checking the motors you can do a case drain test, There are 2 high presure lines going to the motor and a smallar one which is the case drain, Remove the hose at the motor and plug the hose. Install another hose on the fitting of the motor and stick it in a 5 gallon bucket. You need to block the machine up, tires off of the ground and disable the lockouts so you can run the drive. Run the side in question and see how much flow you get out of the hose into the bucket. You should get a steady trickle on a good motor if you get a steady 3/8 stream the motor is leaking internally. Check the motor in both directions. But check for the stripped shaft first.