1. Driver Input: When the driver presses the brake pedal, it pushes a piston in the master cylinder.
2. Hydraulic Pressure: This action increases hydraulic pressure within the master cylinder, a reservoir containing brake fluid.
3. Pressure Distribution: This pressure is then distributed through steel brake lines to the wheel cylinders (for drum brakes) or calipers (for disc brakes).
4. Wheel Cylinder (Drum Brakes): In the rear (likely), the increased pressure pushes pistons in the wheel cylinder outward. These pistons force brake shoes against the inside of the brake drum, creating friction and slowing the wheel's rotation.
5. Caliper (Disc Brakes): In the front, the increased pressure forces pistons within the brake caliper to squeeze brake pads against a rotating disc (rotor), again generating friction and slowing the wheel.
6. Brake Booster: A vacuum booster assists the driver's effort, significantly reducing the force required to stop the vehicle. This booster uses engine vacuum to amplify the pressure created by the master cylinder.
7. Proportional Valve (Often Included): This valve helps to adjust the braking force between the front and rear axles. This is crucial for preventing rear-wheel lockup, especially during hard braking.
8. Anti-lock Braking System (ABS – Optional, but increasingly common): If equipped, the ABS system uses sensors to monitor wheel speed. If a wheel begins to lock up, the ABS module will rapidly modulate the brake pressure to that individual wheel, preventing skidding and maintaining steering control.
In short: The system uses hydraulic pressure generated by the driver's foot, amplified by a vacuum booster, to squeeze brake pads or shoes against rotating components, slowing the vehicle. The inclusion of a proportional valve and potentially an ABS system enhances safety and control. Always consult your owner's manual for specifics about your particular 1999 GMC Sierra model's brake system. There might be variations depending on the trim level and optional equipment.
