Joe Blow Pump Maintenance
What precisely is wrong with it?  If it's got high friction when you work the pump without it connected then working the right kind of lube into the cylinder would do the trick.  (Not sure what the "right kind" would be, but definitely NOT regular oil.  Probably either a glycerine product or a silicone product.)

If it's leaking air back through the cylinder then lube might help, or you may need a new "leather" (which is no doubt plastic/rubber).  

If it's leaking through the "chuck" that fits on the stem then some lube there may help, or a new gasket.