Vacuum canister troubles
 

Hello, forgive me if I should have posted this elsewhere. I am new here but have found this site to be incredibly helpful in the past:

situation: I have a 86 k5 with a carb'd 350. Everything is stock. It runs but is very sluggish and rough idle. spark plugs and wires and such are all new and the timing is good. My dilema is most interesting to me as I am not much of a troubleshooter with engines. In the front center of the engine bay lies a small vacuum canister just left of the alternator, there are 3 hoses connected to it. The very top vacuum line is one of the small plastic lines no more than 3\16" in diameter.

Problem: when I disconnect that line, I get vacuum coming from both openings. Vacuum from the disconnected line as well as vacuum from the canister nozzle where the line connects to.

How in the world can that happen???? Any help is so gratefully appreciated, this is my daily driver and I am trying to get it to pass smog. Thanks in advance!