I have a new pool installed in Aug 2012 and I have been noticing that when you switch the 3 way valve to full main drain, the water flow is roughly cut in half and there are lot of bubbles in the pump strainer basket and it sounds like cavitation. When I switch back to the middle or the skimmers, it's fine. Is this normal or should they both flow the same? I have always noticed the sound difference, but when I backwash, I can see the difference. I wonder if the line has some rocks in it or some other obstruction? I plan on re-plumbing the whole pump pad because the PB did a terrible job and would like to correct this, if possible.