Picked up a couple of Benjamins (310 and a 342) that had been set in a shed and forgotten about for the last decade.
None would take air, so remembered the old advice about adding air gun oil (figure you've got nothing to lose as the thing already doesn't work). Ran in seven or eight drops into the intake and about the same ran down the barrel to get to the outlet, and let it sit standing upright in a warm corner for a day.
Pumped hard and fast, they'd take air, hold it, and shoot. Pumped slow, they'd get no air...which pointed to the seal on the pump rather than the inlet or echaust valve.
Little bit of fiddling and cleaning, soaking the offending parchute seal, and they both run just fine.
Surprised me...it will spue oil at the fist shots like a breeching whale, but it seems to do no harm and certainly broght both of these back from the dead (and i suspect both will need a rebuild before long, but it's nice to get them running with no real $ out of my pocket).
---Along with that old 310 came 4 tins of Benjamin lead round ball. Figured ti wuld be a pretty inaccurate gun, smooth bore...but once I figured out (1) use the hollow of the bolt probe to hold the ball and (2) the more pumps, the worse it shoots, it's doing pretty good work out to 10 or 12 yards.