Back in the day, we farmed and after lay-by, I worked at a service station. Trucks, cars, trailers, tractors, log trucks, 18 wheelers - standard rim and split ring. (I hated that damn pogo stick) We always used glue. The glue was livestock based. Horse/cow/mule and - for Gumbo - donkey. Anyway, it worked as a lubricant and a sealant. You might try that. Presumably your rim is not damaged or rusty and the seating land is sufficiently wide. I know you already checked the valve stem so that is out. Good luck.
