I have what I think is a fleck 5600. Timer is not advancing, so I ordered a new motor, to arrive late next week. However, when opening it up, I have noticed that there is no piston installed on the unit. The hole is there, but sealed from the inside. The unit has worked fine and has been serviced by a company in the past. I'm curious why the piston has been deleted from the system and if it should be reinstalled, should the motor fix not do the trick.