as I said earlier , its possible that the position of the cable is holding the switch on , then when pressed , the cable is passing the switch contacts and turning off
as posted before , remove cable at both ends and pull at brake drum end till off (bulb), then pull at lever end to see if it goes on

I would be very suprised if venhill had fitted a normaly on switch , its more likely that the cable has been reversed or set in wrong place