The function of the switch can be checked by actuating it up and down with your hand. Grab the switch where it enters the rubber hose. If the motor turns on in both directions then the alignment of the assembly should be checked.
ACCESS:
The plastic panel will need to be removed from the top of the base for pedals located toward the end of the base. If the pedal is toward the middle of the base then the section of plastic skirt which is attached to the steel base with screws will need to be raised for viewing.
DESCRIPTION OF ASSEMBLY:
The stem on the switch has three positions, up, down, and neutral. When the switch is in the neutral postion, the black hose should be straight and both springs should have even tension on them. Photo 1 The pedal should be parallel to the floor. Photo 2.
SOLUTION:
Most likely the pedal is favoring one side or the other. Loosen the bolt on top of the collar that has the two springs connected. The collar will self-center itself between the springs when the collar is free. Make sure the pedal on both sides are parallel to the floor and the black hose is not putting pressure on the switch either up or down.
Tighten the bolt in the collar where the two springs join.