VACUUM CHECK VALVE
(1) Remove the vacuum check valve. On gasoline engines, one valve is located in the vacuum supply tube (black) at the intake manifold tap on the right side of the engine. A second check valve is located next to the tee fitting in the vacuum supply tube (black) near the dash panel in the engine compartment.

Make sure you check the condition of the reservoir too. If it's cracked / leaking it wont hold a vacuum during high load conditions:
The vacuum reservoir is mounted in the passenger side cowl plenum area, under the cowl plenum cover/grille panel. The cowl plenum cover/
grille panel must be removed from the vehicle to access the vacuum reservoir for service.