Chipmunk, We use to call that cannister a "dead air chamber." All it did was buffer changes in input or output much like a capacitor does in power supplies. Sort of like having a compressor with a 25 gallon or 50 gallon tank. The compressor will cycle more often with the 25 gallon one. So, the bigger your cannister the less change you will see in wiper speed for the same amount of change in vacuum input. "On the road again"...I don't believe it's the amount of air but the speed of the air and the differential pressure drop across the venturi.
