It's been my understanding that the vacuum from the pump was to aid the wiper motor function when you stepped on the gas and manifold vacuum dropped. That way, you would still have wipers working.
I can remember, hate to admit it though, driving some cars that whenever you'd step on the gas the wipers would slow up, almost stop,until you eased up on the gas.
Kind of a built in speed governor for rainy conditions when you don't need to drive fast.
Ya, I'm kidding about the governor part.
Dan
