Umarex XP 40 review video

Suggestion that comes from Tom Gaylord on the CO2 pistols (and rifles, for that matter) is to put a drop of Pellgun oil on the top of the fresh CO2 canister before inserting it into the nipple on the gun. The oil will be atomized and spread throughout the gun's mechanism, coating and refreshing the seals. Others in the industry echo this advice. Tom in particular attests to having resurrected a number of CO2 guns this way. May help with the leaky nature of the Umarex you have in hand. Let us know if you find that advice to be of any help.
 
I will say that the pellgun oil is a bit spendy.....if you do some looking around you will find all kinds of recipes for home made oils. I am pretty lazy and just use silicone motorcycle shock oil fluid....also synth transmission fluid is also a good choice. On anything new you just want to stay away from Dino based oils....the seals in the new guns don't like it.