Blower will switch from the vents to the defrost vents

Find the vacuum line that comes out of the firewall, this is the supply line, and it goes to a small reserve tank in the engine compartment or under the das, then anothe line goes from the tank to a check valve, then to manifold. If the reserve tank cracks, or the supply line breaks, air will divert to the defrost vents when accelerating. The reserve tank supplies needed vacuum to A/C system during these times! Mot expense, usually the check valve or tank...here's a vacuum schematic: