the engine is generating a lot of vacuum, but do not allow air back in. Vacuum canisters only allow vacuum accessories to be operated for a very short time, and air will leak in after the engine turns off. The most ubiquitous vacuum-powered accessory is the booster for the power brake system. The vacuum is only an assist and the brakes can still function, requiring greater force, if the booster vacuum is used up. Many older vehicles used vacuum-powered windshield wipers. Loss of manifold vacuum when the engine was working hard, or at wide open throttle, necessitated using a vacuum booster pump which was usually part of the fuel pump. Automotive vacuum systems reached their height of use between the 1960s and 1980s. During this time a huge variety of vacuum switches, delay valves and accessory devices were created. As an example, a 1967 Ford Thunderbird used vacuum for: Power brakes Transmission shift control Doors for the hidden headlamps Remote trunk latch release Rear cabin vent control Power door locks Ventilation air routing Control of the heater core valve Tilt-away steering wheel release Such systems tend to be unreliable with age as the vacuum tubing becomes brittle and susceptible to leaks. Pneumatic Pneumatic (compressed air) systems are rarely found in passenger cars. Larger vehicles often use air brakes and the pressure may be used to drive other systems. Windshield wipers, automatic gear boxes, and other common hydraulic or vacuum powered accessories are often adapted. On buses where the engine is often at the rear of the vehicle, compressed air may be used for the throttle and clutch. Bus doors are typically air powered, as well as the steps and the suspension, allowing the bus to lower itself or "kneel" at stops to allow passengers on or off. References Automotive electrics