Vapor Canister Operation and Inspection

From a Corvette Forum post by SWCDuke. Thanks Duke!

The vapor cannister has a removable cap with a spring and diaphram inside. This is the signal valve. It is connected to a "ported" orifice in the carburetor that applies vacuum and opens the valve when the throttle plate is opened a bit. You can pop off the cap and inspect the valve and diaphram for physical damage, then put it together. With no vacuum on this valve the purge line should not flow. Then place a vacuum on the signal line and the purge line should flow.

The canister is basically very simple and the same one was used on virtually all GM cars for many years. The only required maintenance is to change the filter at the bottom of the canister once in a while. I recently bought a Purolator brand filter at Pep Boys for about a buck-and-a-half. It is identical to the original.

The nipples marked "fuel tank" and "carb bowl" allow the activated charcoal in the cannister to absorb fuel vapor, and while driving, the signal valve is opened allowing fresh air to flow through the filter and charcoal to purge fuel vapors, which are drawn into the inlet manifold - either directly or through the PCV plumbing. The charcoal should last forever, and the only required maintenance is an occassional filter change. Whenever I change the filter I check the purge function also.