I have dissasembled, cleaned, and reassembled the vacuum wiper motor to my 26 DB. When I apply vacuum, it moves only in one dirction and then stops. It has plenty of strengyh when it moves the arm. When it stops, there is very little resistance when I move the arm back the other way until it gets to the stop and then the motor returns the arm with good strength. The motor is an Anco Maxi-vac model #MV-1b Should there be a return spring or is there another problem?