Vacuum Reservoir Purpose

I have never seen a II with a vacuum reservoir before?? All 5 of my II's were V6 and V8 cars though. A few of my cars came in a basket so who knows what they may have had though Far as I know the only reason for a vac can is to keep a constant high level of vaccum to run vac accesories like the brake booster. When dudes put in monster cams in their engine they run out of vacuum to run the booster.
If your power brakes work well at an idle drive on!

The brake booster has it's own reservoir which, when these cars were new, was good for more than one application of hte brakes. IIRC it took 3 pumps of the brakes to completely neuturalize the booster.

You won't have a vacuum reservoir can if you don't have AC. The can was there because the heater controls work from vacuum, the control valves are notorious for seepage, and without the can to keep the control circuits supplied with low pressure on long pulls the heater/AC box moves to it's default position which is defrost.

If you can do a longish pull without the heater box changing position you have a true rarity in IIdom: air tight control circuits.

A/C! I was thinking about that because I never had a II with A/C! My 76 Cougars had A/C and vac cans.

Glad I could be of help.