Did it ever work as designed ? If it did and now its not I would bet your solenoid shot craps. And if its wired to energize with the brake lights you are probably not purging the canister long enough,plus running rich @ stops. Most canister systems purge when the engine is cold so as not to cause any warm engine richness. The early systems used a simple temperature vacuum switch that screwed into a coolant port @ the intake manifold,normally open (purging) until engine hits 180 or so.