Customer's Pentair 320 drains when pump is off then doesn't refill


In The Industry
Jul 3, 2013
My customer has a Pentair 320 inline chlorinator. It drains of water when the pump cuts off, and doesn't refill when the pump is turned on unless you open the lid. Am I correct in thinking there could be 2 issues?

1: the O-ring in the lid could need replacing

2: the check valve needs replacing?

I think the check valve is marked as number 6 in the above illustration. Is that correct?