GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 Yukon Denali 🢒 Brakes 🢒 Hydraulic Brakes 🢒 Master Cylinder Diagnosis

These tests will not find all master cylinder malfunctions. If you cannot find the cause with these tests, refer to Hydraulic Brake Diagnostic System Check .

Visual Inspection
    • Check for a cracked casting or brake fluid leaks around the master cylinder.
        A leak will have at least one drop of fluid. A damp condition is normal.
    • Check the pedal linkage for binding and for incorrect pushrod length.
        If you do not find improper brake pedal travel or brake pedal binding disassemble the master cylinder. Refer to Master Cylinder Overhaul .
    • Check the master cylinder for swollen or elongated primary piston seals.
        If you find swollen seals, refer to Brake Fluid and Brake Fluid Handling .

Low Pressure Check

This procedure inspects for the following conditions:

    • Internal seal damage
    • Porosity
    • Component bore integrity
  1. With the ignition switch in the OFF position, pump the brake pedal in order to relieve the vacuum.
    2. • Pump the brake pedal 5 times for vacuum-assisted brakes.
    • Pump the brake pedal 25 times for hydraulic-assisted brakes.
  2. Apply the brake pedal slowly, approximately 3 mm (0.125 in per second). This generates pressure in the master cylinder.
  3. Gradually increase pedal force. Observe for gradually increasing brake pedal travel (Falling Away).
  4. If the brake pedal tends to Fall Away, perform the High Pressure Check.
  5. If the High Pressure Check does not locate a leak, then the master cylinder has an internal leak. Refer to Master Cylinder Overhaul .

High Pressure Check

This procedure tests for the following conditions:
    • Leaks at the following locations:
       - Connections
       - Hoses
       - Casting
    • Gross internal leaks
  1. With the engine running, apply the brake pedal slowly, approximately 3 mm (0.125 in) per second. This generates pressure in the master cylinder.
  2. Gradually increase pedal force. Observe for gradually increasing brake pedal travel (Falling Away).
  3. Check for evidence of external fluid leaks. Examine the following items:
    4. • The ground
    • The pipe connections
    • The tires
  4. If you observe fluid around any component, that component has failed. Replace the component.
  5. If you find a leak at a connection, tighten the connection to specification. Refer to Fastener Tightening Specifications . Perform the High Pressure Check again.