I/g vinyl liner 4 returns 2 skimmers and 1 main drain. Hayward pro sand filter and super pump 1hp. 2 valves at suction side, 1 between skimmers and 1 between skimmers valve and main drain. pump primes perfectly with no air at all when only pulling from skimmers but as soon as I move the main drain valve like three "clicks" the pump pulls air and turning valve any further than that and the pump looses prime almost instantly. I cant see the bottom of the pool but have mucked it and am only assuming there is nothing blocking it in the pool itself. I also don't detect that the main drain is connected to one of the skimmers. am I looking at a leak in the main drain line? the pool water level doesn't seem to change as though there was a leak but who knows right? oh, I also get no signs of ANY water when im pulling only from the main drain, once the skimmer lines are closed it dries up like the sahara, tried slowly suctioning air from main drain line to no avail,