mine did this after I left it in storage too long and the water in the bowl evaporated.
I messed with it and had no luck. Tried commercial valve seal in a bottle form thetford, and a few other things.
I ordered a new seal kit, but before doing the job I tried soaking the thing for a few days by filling the bowl with some CLR or something similar, followed by some aggressive opening and letting it snap closed.
That was a few years ago. Haven't had a problem since.
Now I'm more careful about pouring some blue stuff in the bowl before storage to reduce evaporation, and adding a bit of water when it does get low
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