[GRutter]

See attached ZERK Patent, which explains operation.

The plunger forces grease past the ball check-valve and into the fitting.

When the plunger is retracted, the ball check-valve seals, and a vacuum is created at the nozzle.

When the plunger clears the barrel, the vacuum that was formed pulls grease back into the chamber, drawing from the grease that is in the barrel, and sucking the cork seal down with it.

The key to a properly operating grease gun is in making certain that the ball check-valve is operating as designed. Be careful what you use to clear out old grease because you don't want to damage that check-valve.