I've got a Caretaker 99 inground pop-up system in my 10 yr old pool, and the heads are no longer popping up. The filter cartridges are clean, and the screen on the top of the distribution mechanism is clean. I just opened up the unit, checked that the gears are debris free, and spinning fine...re-assembled the system and have 20psi to 30psi going through the system as I dialed back the auxiliary returns, but still none of the 5 zones are showing any signs of bubbling, or water flow. Any ideas? Do I need to replace the 5 pistons perhaps? The pop-up heads seem to move freely when checked them manually.