Muggsy, the pump boosts vacuum, but doesn't supply it. A line runs from the manifold to the pump, and the pump dampens out the fluctuations when the throttle is opened under load. Some applications also had a reserve tank, with a one-way valve to provide a little backup on a long pull. Trucks with the hydrovac brake boosters usually had a big vacuum reservoir for that purpose.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!