Most vacuum assist power brake units have _______ reserve stops.

• A. 1 to 2
• B. 4 to 5
• C. 2 to 3
• D. 3 to 4

Answer: (C)

Vacuum assist power brake units rely on engine vacuum to multiply pedal force. When vacuum is momentarily unavailable—such as after the engine stalls or during rapid repeated braking—the booster’s internal reservoir provides a stored supply of vacuum. This lets you make a few additional brake applications with reduced braking boost. Most units are designed to deliver about two to three reserve stops before the pedal becomes firmer and braking effort is diminished. So the typical answer is two to three reserve stops. If the vacuum source is fully exhausted, you’ll feel the pedal harder and have less braking assist until the engine restarts or vacuum is restored.