I've bought three new A/C vacuum switches and I cannot get it to close when the engine is cold to open the flapper valve to route the intake air over the exhaust. Has anyone had success getting this to work? When I connect the vacuum port on the manifold directly to the flapper it opens it as it should. But whether hot or cold the vacuum switch in the air filter box does not allow the vacuum to pass through - it just leaks out. Help!