emissions device

[rapidride3]

76' 1/2 ton cheyenne 350 orginally a 2 barrel. There is a black plastic round (it's not the vapor canister) device at the side of the a/c core on the firewall. there is one vacuum line coming from it going to the tee where the modulator is at the back of the intake. I was wondering what this device actually is and what it does??? thanks in advance. Matt.

[piecesparts]

It is a vacuum canister. It holds a small amount of vacuum to keep the internal components in the cab from actuating when the engine goes through different changes (acceleration and climbing hills, etc...). It is just a small tank, that is all.

[rapidride3]

thanks alot,,, i guess i should keep it in service on my truck for now since it may have something to do with the defrost/air cond./heater ,,,maybe when i switch the selector for whatever vents/doors to open or shut??? Thanks for the expertise. I had no idea???