Fortunately, when the brakes are applied, you take your foot off the gas pedal and place it on the brake pedal. Maximum vacuum with the throttle closed. Wiper operation, is also at maximum under the same conditions. How ever,....going up hill stopped mine(the wipers) even back in the day. Alternative is to hook the wipers up to a two stage fuel pump. Leave the brake booster to the manifold. Or convert to electric wipers. That and the possible addition of a vacuum canister to the wiper system to slow down the loss of vacuum under load. Those are all the things I can think of. Maybe others have some suggestions.

Steve H