That's a vacuum-assist unit used to help out the mechanical brakes, similar to the ones on my grandfather's 34 Chevy flatbed trucks when they were hauling bottled beer from Milwaukee to Nashville Tennessee in the late 1930's. Later, they added an insulated trailer behind one of their rigs to haul ice-chilled keg beer. No brakes at all on the trailer!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!