Hello, for the past few years I have been chasing down this issue and can't figure out what it is. I have asked my pool guy to take a look and he has given suggestions but doesn't seem like he knows the equipment issues that well. This is an in ground pool with a pentair pump. Pool only with so spa or valves. What he suggested was to put a plug in the skimmer tee section of the pipe that runs down to the drain so it blocks that off. I gave that a try and no luck. I asked him about changing the pump and he said it's still fine.
The issue I am having is the pump will not prime with the vacuum attached. Works fine as a skimmer and the filter reads about 30psi. As soon as I plug in the vacuum it drops down to 5-6psi. So far what I have tried is replacing the vacuum and hoses, changed the basket seal on top and a few seals inside of it. Didn't quite do a full rebuild though. Changed the seals in the backwash valve, Changed the filter panels. The only way I can clean it is with the backwash valve open but suction is very low. After I suck up all the dirt in the bottom and switch it back to normal operation the outlet blows dirt back into the pool for a few minutes. I have to hold a vacuum hose onto the return outlet for a couple of minutes so the dirty water flows out of the pool and not back in. I think that's due to lack of pressure.
Somewhere it is sucking in air and the last place I can think is the suction pipe running from the skimmer to the pump under the concrete. Anyone have any ideas where to look next?