Gravity helps the gear shift lever drop out of second gear. You say it stays in second when you unhook the canister. If you drive the car and apply slight upward pressure when in second gear and it stays in gear you need to adjust the vacuum shift adjustment on the piston by screwing the adjustment out.The adjustment is the fine threaded shaft connected as an assembly of the vacuum line in the centre of the cannister.I would drive the car, shift to second, if it jumps out of second stop, climb underneath and unscrew rod 2 turns and carry on driving to see if it stays in second.