The 880 isn't a CO2 gun, its a multipump pneumatic, so it isn't a gas absorbtion problem. The problem is one of two things- Either the exhaust valve stem seal is leaking, or the check valve is leaking. Pump it up and stick the muzzle in a bowl of water. If the stem seal is leaking, you'll see bubbles coming from out of the barrel. If not, its probably the check valve. Pump it up and let it sit a minute or two, then open the pump arm again slightly with the gun upside down. If the arm jumps out or slowly rises on its own than the check valve seal is leaking.
Putting a drop or two of oil in the gun may help for a while, but it probably won't last. If its a new gun chances are the seal is bad. Return it and get a new one.