I have a 1951 3/4 ton Chevy with the original vacuum wipers. When I turn them on, the wipers only go one way. I have to turn them off again to get them to the other way. My parents tell me that they used to drive vehicles where that's how the wipers were supposed to work (you had to keep turning them off an on to get them to go back and forth). Is my truck one of those types of vehicles, or is the wiper motor not working correctly?