Well pressure in the crankcase is usually due to blow by which, if you hadn't done all this work, would usually imply worn pistons and bores. You only changed one piston so I have to assume there was a good reason for not doing the others. You didn't mention a rebore so again I have to assume that there was no problem there. But there must have been wear or you wouldn't have changed the rings, unless you just thought you might as well do it while you were in there so to speak.
What was the reason for the rebuild in the first place? Cam chain snapped or summat?
It maybe that the pistons/rings/cylinders need to bed in a bit. Maybe you are getting a bit of blow by due to an imperfect seal, but usually after a rebuild things should be tight rather than not.
Can you do a compression test and/or a leak down test?
Might help narrow it down.
As for it "choking", what exactly do you mean by that?