Greg Bender
http://www.thisoldtractor.com/gtbender
There is a little ball inside the accelerator pump that serves as a check valve. The ball is solid and made out of some sort of plastic that is very resistant to nasty checmicals. I've even accidentally let them soak in carb cleaner only to find no damage. They seem to last forever, but if you need to check yours to see if it has worn, the size should be 3.175 mm or 0.125" or 1/8".