On my 76 Maverick I noticed that when I accelerate that the vents to the inside of the car close and the A/C comes out the defrost vents until I am no longer accelerating. I was told that I need a vacum canister. Does anyone have one of these? Is this the problem? I have a plastic valve on the one vacum line but it is not working.
All the heater /ac controls are vacuum operated. The canister is designed to maintain enough vacuum to keep it operating when the engine is under load and producing less. You should have a canister like the one on the right but it should have 2 lines going to it. One input, which runs through a one way check valve, and 1 output which goes into the firewall to the control switch. Even if you do have a canister, the system must be leak free and that check valve needs to be functioning.
I think the larger the better. They are common on most cars of our era. You probably have some sorta vacuum leak, so I would chase that first, as it is the problem. If you have modded the car, it may not be making enuf vacuum, or if you can't find the leak, the canister, anywhere on the affected line, will be a vacuun reserve.
