This is a commom problem with the evoloution series skimmers.The flanges become warped with age and the edges where the bolt holes are ,hit before the area where the o ring sits does ,creating too large of a space for the o ring to be clamped tight enough to make a good seal.these flanges need to be machine flat again.put a straight edge across the flange and look at the space beneath it at the o ring area,if its greater then the space at the edges where ther bolt holes are then the flange is warped.If its just the flange on the collection cup side you can get a new collection cup from gary for a small fee.a temporary fix would be to wrap 1 inch teflon tape carefully around the o ring(6 or 8 wraps),making it larger