ewillis
SILVER Star
It does that because the checkvalve is sticking. I rebuilt mine by pulling the canister and taking it apart. Mine took alot of carb cleaner and air to make it not sticky. You can't really do that without opening the canister.
Never failed one with a defective CC, but just passed one in Cali with a questionable one! The hungover smog guy actually backed out with the sniffer tool still in the exhaust!