I just saw your post, gwbisho, and apologies for not responding sooner. Yes, you are right, it looks like your son's pentair check valves had the same issue as mine. However, I am not sure why the valve that was placed before the water heater was mangled the same way as the one located after the water heater. In my case, the valve was located after the gas water heater and before the Pentair Trichlor Tab dispenser (between the two) and the consensus was that the slow water flow may have caused the slow moving hot water causing the rubber material to mushroom like in the pictures and if you re-read post #18, and look at the mangled valve picture closer, you'll see the spring melted the plastic part where it came in contact with it in the back of the gate - which is indicative of a heat issue. This is most probably due to slow water flow that caused the water to get too hot when passing through the gate valve. I just hope that the chlorine from the Trichlor Tab dispenser did not cause damage to the gas water heater's copper dispenser unit.
During that time frame, Pentair suggested the water flow to be ~1800 rpm or more when heating, but I had set the water flow at 2200 rpm when heating - which is faster. But now, my PB's service technician advised me to set the water flow to be no slower than 2600 rpm. I've also replaced the pentair check valve with a Magic CR2 valve -> Magic Plastics | “CR2®†Corrosion Resistant Check Valves - that is completely corrosion resistant protecting expensive Heat Exchangers from the potential damage of inline Chlorinators, and tolerates higher heat temperatures. I also installed a FlowVis check valve -> Flow Meters before the gas water heater so I can monitor the water flow in GPM rather than RPM.
During that time frame, Pentair suggested the water flow to be ~1800 rpm or more when heating, but I had set the water flow at 2200 rpm when heating - which is faster. But now, my PB's service technician advised me to set the water flow to be no slower than 2600 rpm. I've also replaced the pentair check valve with a Magic CR2 valve -> Magic Plastics | “CR2®†Corrosion Resistant Check Valves - that is completely corrosion resistant protecting expensive Heat Exchangers from the potential damage of inline Chlorinators, and tolerates higher heat temperatures. I also installed a FlowVis check valve -> Flow Meters before the gas water heater so I can monitor the water flow in GPM rather than RPM.