Bill -
That is your "vacuum tank" for the windshield wipers. Those items are difficult to find as they tend to rust quickly or disappear for whatever reason. Consequently they are not seen or used that much. Instead a brass fitting is used in it's place and it will also work to propel the wipers. This little tank, however, actually stores up a small amount of vacuum to keep the wipers more steady during a driving in a rain event. I have often thought that a larger vacuum tank would work even better, but our engines apparently do not produce enough vacuum to store a large amount.
Joe
