I am glad the steel nipple solved your problem. But I am still afraid you have a heat problem. The water should not get hot enough to soften PVC pipe. I only use the steel nipple in the jet pump pictured to hold the weight of the tank. Otherwise I would use PVC pipe if the tank was mounted on the wall.
You can check the minimum flow to see if the water is getting hot. Just open a hose until the pump comes on. The reduce the amount of water coming out of the hose to a trickle. If you have a 40/60 pressure switch, adjust the output of the hose until the pressure is steady at 59 PSI. This should be the minimum flow the CSV will allow. Measure the amount coming from the hose in a bucket and a stop watch. And feel the pump housing to see if it is getting warm.
If the low flow is less than 1/2 GPM and the pump is getting warm, you can reduce the pressure switch setting. Shutting off the pump at 55 instead of 60 maybe all you have to do to keep the water from warming up.
