Great thanks Bob. What is inside the can? ...................
Nothing is "in the can". Its simply a reservoir for the vacuum (a place to hold it until its needed) needed to operate switches, plungers and other items that push and pull things.
Not sure that I would try and open one up. Perfectly sealing the fitting to the can again might be difficult.
Possibly - if it makes allot of sound when you shake or rotate it - you might inject some evapo rust into the insides to kill any rust and coat it a bit. For me if it doesn't need it focus on the exterior, making sure its solid and making it look original again
NOTE: Merged about five posts from the following thread over to this one
Blue Primer under sound deadener outer front fenders San Jose 1968