If all the parts check out and the pump is doing very little you may need to flush the pump. The vanes on most of these oilless pump get packed with powder eventually and needs cleaning out. This is done outside in a well ventilated area, with the pump running slowly dribble some solvent like naptha (never gasoline or similar types) into the inlet side of the pump. It will blow out the outlet end, it may be necessary to remove any hoses or muffler parts from that end. Do not allow any open flames or sparks nearby. Usually the pump will start working soon after the flushing, sometimes it needs a second flush. If this doesn't help the vanes may need replacing, a dirty dusty job but not beyond the experienced shade tree mechanic.