Finding pin holes in a closed loop system

[midgetwaiter]

It's pretty unlikely that you have something like a pinhole in the side of a pipe, you just don't have enough pressure to rupture PVC. If you have a hole it will be somewhere at the edge of a fitting where the glue has a space in it. This space could theoretically allow air to be sucked in by the movement of the water through the pipe like the venturi on your skimmer works.

In practice this is pretty unusual in a return line, the back pressure would be more likely to force water out of the hole than draw air in. Due to the rise in your system I imagine you have a check valve on there that will keep the pipe full with the pump shut down. Turn off the pump and look for leaks either in the plumbing or around the pump volute.