GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Sierra - 4WD 🢒 Brakes 🢒 Hydraulic Brakes 🢒 Master Cylinder Diagnosis

Visual Inspection
    • Inspect the master cylinder for a cracked casting or brake fluid leaks.
        A leak will have at least 1 drop of fluid. A damp condition is normal.
    • Inspect 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 .
    • Inspect the master cylinder for swollen or elongated primary piston seals.

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 (1/8 in) per second). Applying the brake pedal generates pressure in the master cylinder.
  3. Gradually increase the pedal force. Observe for gradually increasing brake pedal travel (falling way).
  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:
       - The connections
       - The hoses
       - The casting
    • Gross internal leaks
  1. With the engine running, apply the brake pedal slowly (approximately 3 mm (1/8 in) per second). Applying the brake pedal generates pressure in the master cylinder.
  2. Gradually increase the pedal force. Observe for gradually increasing brake pedal travel (falling away).
  3. Inspect for evidence of external fluid leaks. Examine the following components:
    4. • The ground
    • The pipe connections
    • The tires
  4. If you observe fluid around any component, the 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.