Hi Rob, welcome to the forum. Take the valve back apart and check the o-ring on the underside of the top, sounds like it got pinched (or worse fell off and you didn't notice) If it's in good shape lube it (NEVER USE VASOLINE TO LUBRICATE A POOL GASKET OR O-RING!!!) and make sure it's seated properly before tightening the screws.
The valve is the obvious place to check - any chance of posting a picture of the gasket? Was any of the plastic in the channels the gasket sits in broken or warped? I'll help in any way I can, keep us informed. Again, welcome to the forum! - Waste
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