Hey Dog and others! RE: turbulence from an obstruction causing air bubbles - yup, it happens, I had a customer 5 yrs ago who had air in his hairstrainer but only when he vacuumed. My first thought was a pin hole in the vac hose, but using ours produced the same result. A pressure test revealed no leaks, however we got lucky and relieved the pressure at the skimmer and a bunch of acorn remains perked up into the skimmer. It seems he had vacd up ~5 lbs of acorns the previous fall with out the skimmer basket in place - they all collected at the first 90. The pertinent point to this story is that if the obstruction is lagre enough and you are only using that line for suction, the turbulance 'cavitatets' the water, hence the bubbles in the pump. This could be what was happening in your pool if you were only using the MD. As for the skimmer being fuller correcting the bubbles, if the obstruction was large enough - I'd be like having the MD valve off or the drain plugged, more work for the pump and skimmer line. What I picture happened was, with the water in the skimmer a little low, the 'extra' suction made the water vortex, sucking in a little air which appears as bubles in the pump (or the suction on that line was strong enough to drain the water out of the skimmer before the pool water could refill the skimmer - same thing happens when the weir is stuck in the vertical position) The only thing that comes to mind on the ? of why more water in the pool (higher on the skimmer) would aleiviate the MD bubbling is that the extra pressure from the water is enough to increase the boiling point of water, it would seem that if my theory is right you were on the margin of having the problem. (That's as scientific as I get )
Kurt(good to see you again,how was your winter?) Some builders do plumb the main drains into the second port of the skimmer and use a valve they place in the skimmer to regulate the 2 suctions(IMHO - very stupid for at least 2 reasons, 1. If the skimmer line fails you'll have no suction from the pool until it is fixed and can't run the system 2. If the water falls below the skimmer, you can't run the pool). However, as there is a valve for the MD, it must be running on it's own (clogged) line.
Carl, I too thought of the dynamite route, but I figured Dog would want some 'closure' on the issue and it would be really hard to identify that particular glob of crete and the pipe it was sitting in amidst all the mud, crete chunks and plastic shrapnel
Dave, I should have had you write my tag line - 'Listen to "Waste"' - I like it