Vacuum line from carb to?

[dan42]

There is a vacuum line that comes from the base of my carb and goes through the firewal and under the dash - does that assist the heater/ac controls? When I have it hooked up, I hear a hissing noise from under the dash, so I'm thinking I have a hole in the line under there somewhere.

[BurnoutNova]

not certain, but if your truck has A/C there may be a vacuum reservior to actuate the mode doors, on gm cars atleast there like a 6" ball under the hood, but ive never seen where there at on the trucks

[jhow66]

Just follow the line and plug the end -no hiss -truck will run better (idle speed will slow down when you stop the leak)

[dwaite72lnghrn]

Not sure what year you have but if it is factory air you will see a vacuum resovoir on the airconditioning housing above the passenger valve cover. One end runs to the "T" on the back of the intake manifold and the hose out the other side runs to the vacuum switch "top lever" of the heat/air controls. The vacuum switches can go bad and leak. attached is the vacuum diagram and a picture of a '72 vacuum canister.