Originally Posted by AdvntureCats
It's a vacuum accumulator, which provides a "reserve" of vacuum for any accessories that might need it when the engine intake vacuum drops too low. These are usually connected to the heater/AC controls so they work at all times, and keep the heater/defroster duct diverter flap in the system where you set it. (On many systems, a spring pulls the flap to direct air to the defroster ducts by default if vacuum is lost.)
Sometimes there's a tee in the output line so the accumulator can also supply vacuum to the cruise control; under wide-open throttle, vacuum will drop -- just when you're climbing a hill and need the cruise control's vacuum actuator to pull harder on the throttle linkage. The accumulator keeps everything that's vacuum operated working as it should.
Just trace the vacuum supply lines back from the HVAC controls and the cruise, if any. If both lines are broken, chances are they may go through a tee splitter to the accumulator.
Back in the day (the Fifties) I was in the USAF and had a 53 Merc flathead with vacuum windshield wipers, when you stepped on the gas or went up a hill they stopped and always at the absolute worst time.
The motor pool fixed their vehicles by taking a bunch of 1 qt soup cans, soldered a small tube on each end of it and put it in the middle of the line between the wipers and the intake manifold source and the wipers no longer stopped or slowed when you stepped on the gas. Everyone tried to swipe a can from a USAF pick up or make one of their own, I got lucky and had a friend in the motor pool and he made one for me for a 6 pak, problem was I was 19 and the drinking age was 21 so I had get a sarge to buy me 6 pak so I could make a swap. To me at the time that was the coolest thing I ever saw