While futzing with my car I decided to check the operation of the carbon canister. The only real function is the vacuum controlled valve the activate the purge cycle (pull vapor from the can to the engine intake vacuum). What I found was the valve would not close in the absence of vacuum at the signal line (ported vacuum small hose). I test it using a hand pump and vigorously pumping. Now maybe in the presence of a constant vacuum source it would "catch", the design of the valve would tend to do