Not familiar with the specific pump, but sounds like a common problem with certain type of AC motor. Usually, there is a mechanical mechanism that prevent the gear to rotate the opposite direction, and for medical application (dialysis,,etc.) you problem will certainly ensure an expensive lawsuit.
The motor/gear is faulty, and see if it is still under warrenty. You may need to return it to the manufacture, but you need to deal with this issue with the seller/manufacturer. If it is medical pump that was refurished and decertified, you are probably out of luck... but it does not hurt to contact the responsible parties. Good luck.
