When it comes to brake systems, the master cylinder is at the heart of it all. It’s the component that ensures hydraulic pressure is sent to the brake lines, allowing the vehicle to stop when you need it most.
But like any other part, the brake master cylinder can wear out or fail, often without much warning. As a brake repair professional, quickly diagnosing a faulty master cylinder is crucial for providing top-notch service.
In this guide, we’ll introduce the best methods for testing if the brake master cylinder is bad, helping you pinpoint the issue and get your customers safely back on the road.
Method 1: Perform the Pedal Test
Step 1: Build Pressure in the Brake System
Start by sitting in the vehicle and parking it on a flat surface. Press the brake pedal a few more times to increase the pressure on the braking system.
This ensures the brake lines are pressurized, so you can test how the brake pedal performs under normal driving conditions.
Step 2: Pressure on the brake pedal
Press the brake pedal down and hold it in place with steady pressure.
If the pedal slowly sinks toward the floor or becomes spongy, this could indicate that the master cylinder is leaking internally or has failed.
Step 3: Evaluate the Pedal Response
If the pedal does not stay firm or slowly sinks, it’s time to inspect the master cylinder for issues.
This is one of the most common signs that your brake master cylinder may be bad.
Method 2: Inspect for External Leaks
Step 1: Locate the Brake Master Cylinder
Open the hood of your vehicle and locate the brake master cylinder.
It is usually located near the back of the engine bay, attached to the brake booster.
Step 3: Verify Brake Fluid Loss
If you notice leaks or fluid buildup around the master cylinder, it’s a strong indication that the master cylinder may be faulty and needs to be replaced.
Method 3: Check the Brake Fluid Level
Step 1: Inspect the Fluid Reservoir
With the engine off, check the brake fluid level in the master cylinder reservoir.
Ensure the fluid is at the recommended level.
Low fluid could be a sign of a leak in the master cylinder or elsewhere in the brake system.
Step 2: Monitor Fluid Levels
If the brake fluid level is low and there is no visible external leak from the brake master cylinder, the problem may be an internal leak in the master cylinder.
You can fill up the fluid and monitor the level over time to see if it drops normally.
Step 3: Watch for Continued Fluid Loss
If the fluid level continues to drop despite topping it up, this is another sign that the master cylinder may be defective.
Method 4: Test for Internal Leaks (Pressure Test)
Step 1: Apply Pressure to the Brake Pedal
With the engine off, press the brake pedal down firmly and hold it in place.
Observe whether the pedal feels firm or if it gradually sinks.
If the pedal sinks without losing pressure, this is an indication of an internal leak in the master cylinder.
Step 2: Use a Vacuum Pump for a More Accurate Test
To get a more accurate test, you can use a vacuum pump to check for pressure retention.
If the vacuum pump can’t hold the pressure, it indicates a problem with the master cylinder.
Method 5: Perform a Bench Test (For Removal)
Step 1: Remove the Master Cylinder
If you suspect the master cylinder is the issue, you may need to remove it from the vehicle.
Secure the master cylinder in a bench vise for a more thorough test.
Step 2: Test the Plunger
Insert a screwdriver into the rear plunger of the master cylinder and press it. The plunger should feel firm and resist movement.
If the plunger moves too easily or feels soft, the master cylinder is likely bad and needs to be replaced.
Method 6: Listen for Unusual Noises or Pedal Feel
Step 1: Listen for Grinding Sounds
When driving, listen for unusual grinding or squeaking noises when you press the brake pedal.
This could indicate that something in the brake system is failing, such as the master cylinder.
Step 2: Test Pedal Feel
Also, look for unusual “softness” or sponginess in the brake pedal. If the pedal feels soft or does not respond immediately, this may indicate air in the brake lines or a faulty master cylinder.
Conclusion
In addition to these ways to test if the brake master cylinder is bad, we also need to test the brake booster because sometimes, a faulty brake booster may show the same symptoms as a faulty master cylinder and can confuse us. If the brake pedal remains hard during engine startup, the booster may be malfunctioning. Perform a separate test to rule out the brake booster as the cause of the problem.
