Power brake boosters use engine vacuum pressure to apply power to the brake master cylinder. 1971-1974 Bus Power Brake 8-1/2 inch diameter Booster and Servo. A brake booster does not need to have as much force exerted on the brake pedal when applying the brakes. The power brake servo mounts in between the brake pedal push rod and the master cylinder and connects to the engine with a reinforced vacuum line.
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