2stroke_shaman
ArboristSite Guru
I used a silicone rubber stopper from a Caplugs kit assortment & stuffed it into the big threaded hole. That also sealed off the little air hole.
Pulled it back out with needlenose pliers. If it goes too far & falls into the tank, you can retrieve it out of the oil filler hole or pull the pump’s sheet metal cover off to get it.
The small perpendicular air hole is what pressurized the oil tank, but you can’t plug it & get the check valve screwed back in. At least I couldn't figure out how...
I think you could use a 3/8-24 x 1/2 fully threaded bolt with a little teflon tape to seal it for the test.