How to Check for a Defective Master Cylinder

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It's been estimated that a typical driver uses the brakes on his vehicle an average of 75,000 times per year. While most drivers are confident the braking system in their vehicle will function properly each time they apply the brakes, the worst time to discover brake failure are moments before a collision or other emergency. One such brake malfunction that has been identified as a contributing factor in several traffic accidents is a defective master cylinder. However, there are simple and inexpensive ways to test the reliability of the master cylinder that will ensure the brakes respond properly when needed.

Skill level:
Moderately Challenging

Things you need

  • An assistant
  • Reliable brakes
  • Brake fluid
  • Safe roadway

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  1. 1

    Inspect master cylinder for indicators of wear. Conduct a visual inspection of the master cylinder for evidence of fluid leakage. Most master cylinder failures are caused by leaks developed around the piston seals. If leakage is visible or damage is present, the master cylinder should be replaced. Continue with testing if no leakage is determined.

  2. 2

    Drive the vehicle a few feet then stop. Drive the vehicle a few feet then hold down the pedal a few seconds to observe how the pedal reacts. A pedal that drops to the floor, otherwise known as spongy, is an indicator of air in the brake line and is a common reason why a master cylinder will malfunction.

  3. 3

    Test for air in the system. A quick test that determines whether or not air in the brake lines is the problem is to pump the brakes 15-20 times. Hold down the 20th pump firmly while an assistant removes the master cylinder cover. If the fluid is jetted upward, then air has entered the system. At this point, you should bleed the lines by following the proper procedure for your particular vehicle.

  4. 4

    Plug master cylinder outlet ports. When the brake lines have been properly bled but the brakes are still spongy or dropping, remove the master cylinder lines and block the outlet ports. The desired result should be a full pedal. However, if the brakes are still spongy or dropping, or if bubbles are present in the master cylinder reservoir, the master cylinder could be defective and require replacing.

Tips and warnings

  • Prior to conducting the tests, make certain the fluid level in the master cylinder is full. If fluid level is inadequate, bleed the brake lines then add the proper amount of fluid to begin testing.
  • Brake fluid on your vehicle will result in damage to the paint.
  • Make certain the brakes are reliable before conducting any road test.
  • Make the certain the road is safe from traffic and pedestrians before conducting a road test.

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