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Is your check valve upside down? or is your ceiling concrete and have a wet spot?
I typically use a 2" pvc union. Is that check valve spring operated or just a gravity flap? The union allows disconnect without being a clamp based fitting.
Don't put any kind of sealing compound on the furnco - rubber doesn't like chemicals.
I repaired a friend's sump and discovered the value of the "check valve" because his system did not have one lol. If it's not sealing though you're sure you have more than tightened the clamp, then the size different between the hoses is too great. Might need a reducer/spacer to go between the backflow valve and the PVC pipe. BUT... it looks like you've actually already done that now that I look at the pic closer lol.
SO... at this point, if it were me... I'd go get some FLEXSEAL or silicone. Take the leaky joint apart, and apply some. re-attach. Wait for cure (24 hours) before testing for leak again. Be aware that FLEXSEAL will permanently bond with rubber (because it is liquid rubber), so be careful where you apply it.