[Al Skierkiewicz]

This compressor is known as a "wobble" or WOB-L (TM) pump. The piston is a single piece integrated with the piston rod and seals. It does not have a wrist pin where the rod connects to the piston. This model is not designed for vacuum so you must use a venturi to produce vacuum using this pump. Thomas does make another pump that looks similar that does produce vacuum. It essentially is the same pump with a different head and valve assembly. That pump is part of the 415 series not the 405 series.
The pump works fine in either direction, but the efficiency is higher in the direction marked and therefore less heat is developed.