Draining a hydraulic pump or motor.
In hydraulic pumps and motors there always leaks oil from the pressure side into the housing. If this oil is not removed, a pressure will be built up inside the housing with the result that the shaft seal is pushed out of the housing! Therefore the prescribted maximum housing pressure (often 2 bar or 0,2 MPa) should not be exceeded. To prevent this problem hydraulic pumps and motors generally are equiped with a drain port. This drainport should be connected directly to the reservoir and the pump/motor should be mounted in that position that the drainport is at the upper side. This to assure that the housing is always kept filled with oil for greasing and cooling purposes . If the drain line has unsufficient capacity , the pressure will increase and the shaft seal shall, as you can see in the animation, be pushed out of the housing as well.