I found this information in a GMC book can anyone help fill in the blanks on this, have you ever seen one? The last truck I purchased (1950 6400) had this vacuum pump booster on the firewall. It also had a small rheostat switch with VAC on the dial in the cab. I have found reference to this in my GMC parts book but not Chevrolet (GMC 23.3787 part #2233295) does anybody know how these worked or any more information on them. I assume it is a 6-volt powered vacuum pump that boosts engine vacuum like the optional fuel pump.