I had some good success with using Liquid Wrench silicone spray on the valve stems of old, sticky PVC valves. I removed the valve handles, sprayed around the stem, then opened the filter vent to create some vacuum....don't know if that helped, but better than pressure. I cycled the valves a few time, let things sit for 30 minutes with pump off, then cycled again. The rotating effort was significantly reduced and they now rotate stop to stop.