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Since you already replaced the gasket you will need to check two others things: First, inspect the groove the new gasket is sitting in as well as the mating surface of the diffuser. Be sure none of the old gasket is still there. Thoroughly clean all surfaces as well to ensure the new gasket is fully seating in the valve. If that does not cure the leak you will need a closer inspection of the mating surfaces to find a crack. Without any pump pressure on the valve it will be tough to find. It will appear as a hair like line but once the pump is on it will open up enough to let water pass. If you find a crack you can try to use a two-part plastic approved epoxy to seal it or else you will need to replace that part.
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John
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