I've lived in my current house about 12 years, lots of experience trying to get my pump to prime and understand the do's and dont's. The problem im having this year is my pump will prime fine if its in backwash or waste or rinse mode but when its in filter mode, it has water coming through ok, just not enough to create suction for my vacumn. Doesn't matter if i have only main drain open, or skimmers open or both, just seems like water flowing through isnt as strong in filter mode. I replaced all the sand and checked the innards on my filter at that time. Also replaced my multi-port valve because there was a tiny bit of leakage around the seal and it was 16 years old. Did all that and still have the same issue. I can visibly look at all plumbing and see no leaks from the pump to filter or any return valves. I do see a tiny bit of water under my filter like it has a leak, but i cant see where it is and would that be enough to cause no this since i do have good pressure/prime when i have filter in backwash or waste mode? I have checked all the plugs in the pump and filter also and added an 0 ring. Not sure what else to check, to get it to prime in filter mode, obviously seems like air leak but not sure where it would be since its ok in backwash mode.