I have air in my pump basket and need some suggestions on how to go about troubleshooting. My pump is new and the seal and drain plugs seem to be fine. Also did not run my pump for 4 days anticipating some investigation work this weekend, and upon "breaking" the union fitting, the water in the system drained - suggesting to me that the vacuum was maintained over the 4 days I did not run. I only have one line going out to the skimmer so it's a pretty simple system. Aside from replacing all the PVC plumbing at the pump - are there simple tests I could do to try and find the source of the air?