I have been opening/closing my inground pool for 15 years. First time I hit this problem at opening. My water seems to be circulating just fine but when I switch off Main Drain to skimmers only I cannot get a good enough prime for the vacuum to work. It does not even fully prime when vacuum hose is not connected. From what I have read it seems I have air in the lines but I do not see any obvious leaks. Any ideas?