To do a leak test, you would need to make a plug that would seal off the gas port in the carrier gas section (read a plastic plug with two O rings, before and aft of the port). Then using soapy water around the carrier/key and air compressor, charge the system. Chances are you might get a few small bubbles from between the key/carrier (fouling will seal these is due time), but if you got big bubbles, you got a major leak that needs to be dealt with.
To solve a leak, you hand lap the key into the carrier using fine lapping compound, then when you go to reinstall the key with new allen screws, you use loctite (not as a bolt locker, but to make a thin gasket between the key to carrier surface to seal any defects in the surfaces that you may have missed during lapping).
Also, before you start to lap anything, take steel counter sink and relieve the two carrier allen screw holes on the carrier (Read remove any high ridges around the openings before lapping or these ridges will just eat groves into the key matting surface), the gas port entrances on both the key/carrier, and confirm that the key/carrier ports do align together (read if they don't match, then enlarge the gas port channel opening of the key until they do (the carrier is hardened and the tougher to elongate).