84677866 | Genuine GM® Brake Master Cylinder Assembly 2019-2022

Converts Foot Pressure into Hydraulic Pressure

• When you press the brake pedal, the master cylinder’s piston(s) push brake fluid through the brake lines.

• This pressure activates the calipers or wheel cylinders, which apply the brakes at the wheels.

Distributes Pressure Between Front and Rear Brakes

• Modern master cylinders are dual-circuit for safety — one circuit for the front brakes, one for the rear.

• If one circuit fails (for example, due to a leak), the other still provides partial braking.

Maintains Consistent Brake Feel and Performance

• The master cylinder regulates fluid pressure so that the brakes respond smoothly and predictably every time you press the pedal.

Stores and Supplies Brake Fluid

• The attached reservoir holds extra fluid to compensate for pad wear and temperature expansion.

• It also allows easy inspection and refilling.

Supports Modern Safety Systems

• In vehicles with ABS or Electronic Stability Control, the master cylinder often integrates with sensors and control valves that modulate pressure electronically to prevent wheel lockup or skidding.

Soft or spongy brake pedal: Air or internal seal failure.

Brake fluid leaks: Often from seals or fittings.

Brake warning light on: Fluid low or pressure imbalance between circuits.