48 chevy truck axle questions

my boss and i just started on a build at work, its a 48 chevy heavy truck. his grandpa used to have a service truck just like it that sits out in the weeds, but was in very rough shape so we found another one that was complete and unmolested. it turned out to be pretty solid. ive never messed with heavy trucks like this before, and it has a two speed axle in the rear. i was wondering there is a device on the passengerside that the brake mc runs to and then it runs back to to the mechanism to engae the axle and back to the differantial, is this some kind of vacume pump? and can these be rebuilt or are they a problem, any suggestions? thanks.

 

The device you are asking about sounds like a hydravac. Its a vacuum assisted brake booster. You can buy parts for them and rebuild them yourself.

 

i was thinking it was something along those lines, thanks, good to know parts can still be found