Need Meercham "air leak" help

I would go back to the shop with it and show them. If that is a new pipe (looks great I must say!) then it should not be doing that. there are replacement threaded inserts that they can put in the shank for the stem to screw onto and they will be able to get it to a good seal.

Another options would be to form a gasket at the crack... a small sliver of Teflon or even rubber cut to the correct size to fit in there would do it.

I would hesitate with wax as I wouldn't want to have it become glued in, but then I really don't have much experience with using wax to fix this...