Originally Posted by 6SPD_FTW

There is a vacuum tank somewhere, built into the intake manifold. It uses a one-way valve to maintain a supply of vacuum for the brakes. There typically is vacuum in the manifold only at idle as that's usually the only time the throttles close (unless the VDC system is cutting power to maintain traction). So, vacuum pressure should be maintained at a fairly constant level.

Late,
Trav