Delrin = a brand name for acetal.
When you mentioned the cone shaped delrin seal it sounded to me like an aftermarket stem+seal,
but now that you mention brass it sounds like the stock seal with flat face, not conical.
I thought this was the part you sanded. If it's an aftermarket delrin stem+seal then it
it could do with some spinning on the valve seat.
If you meant the check valve as the conical one, it's made from PTFE (teflon) which
is a lot softer than acetal. It conforms easily to the seat and doesn't need spinning.
Yes, if it's leaking out the barrel then the exhaust valve is not sealing. With the stock
seal the usual culprit is dirt, could also be a perished seal if its old or has been
exposed to solvents. Check the seal face for dirt or nicks, as well as the valve seat.