Fred,
I did not know what it was until recently myself. On most "post stovebolt" vehicles many systems run off vacuum and a means to retain some vacuum when the engine was off was necessary so they developed vacuum cans or balls. On late 70's mid 80's GMs there is a round plastic "ball" mounted in the engine compartment to which several vacuum lines may be connected. If my description is dead wrong then please someone correct me but this is the jist of the matter as it was told to me.
The only things I see on my truck that will use vacuum are:
Transmission modular, Power brake booster, Distributor, and wiper motor (unless an electric option comes about).
