Vacuum wipers slow down, or usually stop altogether when the vehicle is pulling a hill or passing. The vacuum booster pump is in line with the intake vacuum to prevent the wipers from coming to a complete stop- - - - -if it works as designed. Most don't. Chevy was too cheap to put in a reserve canister with a one way valve to be a backup for the intake vacuum.
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!