Clean all the parts up with any good product,, the rubber bits all need replaced while your in there,,, then put it back together and go. should be o-rings and maybe a dust seal at the plunger connection to the lever.
They are a pretty simple gadget. Just look at the bore and piston for any bad scratchies or gouges..
Guns are like parachutes
If you need one and don't have it
You will never need one again