There is a valve on the inlet pipes in the engine bay, under the inlet manifold. It's operated by a tiny vacuum servo.
You'll probably find the tiny actuator arm has corroded and is seized in the guide tube and the system doesn't have enough grunt to over come it.
Don't just try and force the arm yourself, by hand - it will just split the plastic guide (you can probably guess how I know

). You'll need to rub down the arm first to remove the corrosion, then lube it up with something that will help it move and protect from further corrosion.