Is this cap before pump causing air leak?

[tetlee]

Hey, moved in 2 weeks ago and this is my first pool, so this is my first post!

When I turn the valve in front of the pump towards the cleaner the pump sucks a lot of air. When it's split between the cleaner/skimmer I get the odd bubble in the top of the pump filter and a few out the returns. I tried submerging the cleaner robots hose but that didnt stop it sucking air.

When it's sucking air there is a knocking from the pipe connecting to this cap, even when its only pulling a small amount of air.

I took the pump cover off to let the pressure out and tried unscrewing the cap. It gave a little but I didn't want to force it.

Could this be the source of the air leak? What is it for?
Am I ok to unscrew it and inspect it further?

Thanks!

 

[tetlee]

Another thought - when I divert it could it be increasing the pressure enough to suck air through the o-ring on the pump cap?

I gave it a good clean and it looked in ok condition.

 

[jblizzle]

I think that is a check valve, but no idea why there would be one on a cleaner line.