If you have a mechanical pump easy way to check it is to actuate the arm and see if it pumps. There are small check valves that can stick if gas get gummy.
Get some hose, attach a piece to the inlet of the pump and the other in some WATER (no need to shot gas everywhere). Hole the pump at the same height or lower than the water and move the lever... it should pump. If not, rebuild/replace.
