The master cylinder used on GM W-Body vehicles is a composite design (plastic fluid reservoir and aluminum body) designed to be used in a diagonally split system (one front and one diagonally opposite rear brake served by the primary piston and opposite the front and rear brakes served by the secondary piston). It incorporates the functions of a standard dual master cylinder plus it has a fluid level sensor and integral proportioning valves. Proportioning valves are designed to provide better front to rear braking balance with heavy brake applications.
The master cylinder fluid reservoir is on the master cylinder and is located under the hood on the left side of the vehicle. The master cylinder reservoir contains enough fluid so it never needs service under normal conditions. A low fluid level sensor in the reservoir will warn of low fluid level. Thoroughly clean the reservoir cap before removal to avoid getting dirt into the reservoir.
Fluid Recommendations & Fill
The brake master cylinder is designed for use in diagonally split hydraulic systems. It incorporates the functions of the standard dual master cylinder plus a fluid level sensor and integral proportioner valves. In a diagonally split system, the left rear brake and the right front brake are on the same hydraulic circuit in the master cylinder, and the left front brake and the right front brake on the other circuit.
The master cylinder is equipped with a fluid level sensor switch located in the side of the reservoir. The switch activates the BRAKE warning lamp if the fluid level is low. Once the fluid level is corrected, the BRAKE warning lamp will go out.
The brake system in these vehicles is a complex system dependent on absolutely clean brake fluid of the proper type. Use only SUPREME 11 or equivalent DOT 3 brake fluid from a clean, sealed container. Do not use fluid from an open container that may be contaminated with water. Using the wrong fluid or contaminated fluid could result in damage to expensive components, or loss of braking, with possible personal injury.
The brake hydraulic system is a closed system. Low levels of fluid may indicate a need for service as there may be a leak in the system. It is good practice to check the brake fluid level during oil changes or whenever under the hood.
The master cylinder reservoir is on the master cylinder and is located under the hood on the driver's side of the vehicle. The master cylinder reservoir contains enough fluid so that it never needs service under normal conditions. A low fluid level sensor in the master cylinder reservoir will warn of low fluid level, which could indicate a leak.
Thoroughly clean the reservoir cap before removal to avoid getting dirt in the reservoir. Remove the twist cap and diaphragm under the cap. The fluid level should be at the MAX fill mark on the reservoir when full. Reinstall the cap and diaphragm assembly when the reservoir is full.
Removal & Installation
Because the vehicles covered by this manual are equipped with ABS, please refer to the ABS Hydraulic Modulator/Master Cylinder removal and installation procedure, located later in this section. Please note that a Tech 1® or equivalent scan tool is recommended for bleeding the system. GM specifies that when any brake component is repaired or replaced such that air is allowed to enter the brake system, the entire bleeding procedure MUST be followed. Unapproved or partial bleed procedures can allow air to remain in the brake system. This could result in reduced braking performance and possible personal injury. In addition, the scan tool is required to scan the entire ABS system for Diagnostic Trouble Codes (DTCs).