I am trying to figure out what the vacuum valve on top of the windshield washer vacuum can does. It's the one on the cylindrical can on the passenger side. It's a 61 FI Big Brake car if that makes a difference. I can apply vacuum to the top fitting and it seems blocked. The bottom fitting seems connected to inside the can. Is there an internal orifice? should the top one leak down? How do I do a function test off the car? jerry