After a 1K miles, the leak has happened again. Apparently the mating surfaces of the cap and hub must not be perfect. i tried to find a new cap but haven't found one so far- the trailer is about 40 years old. The service guy recommended caulking the cap, but I can't see how that will stick with the grease coating the surfaces and/or infiltrating under the caulk.
So I came up with this idea until I can find another cap. it's a seal ring made from a bicycle innertube. I cut a length about four inches long, and stretched it over the cap and hub, and then doubled it over. It was a bit tough but manageable, and it's tight. I'll clean up the wheel and keep an eye on it for the rest of the season.