Try modeling it as a torques around the pivot point.
For extension:
Code:
How much torque would the weight of the robot on the mecanum wheels impart?
vs.
How much torque can the force from the piston impart?
For retraction:
Code:
How much torque would the weight of the robot on the traction wheels impart?
vs.
How much torque can the force from the piston impart?