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How Do You Test a Brake Master Cylinder?

Brake fluid master cylinder installed on a vacuum brake booster

Your vehicle’s braking system is essential to maintaining your safety and that of other drivers on the road. While a fault in any one of its components can lead to partial or total brake failure, the brake master cylinder is the most important part. You can ensure yours is working correctly by testing how it’s functioning. So, taking into account disc brakes, drum brakes, electronic stability control and anti-lock braking systems, how do you test a brake master cylinder?

Signs That You Should Test Your Brake Master Cylinder

When you step on the brake pedal, the pushrod contacts the primary piston, generating hydraulic pressure. This pressure does two things: It moves the secondary piston and half of the brake system, usually one rear brake caliper or wheel cylinder and the opposite front brake caliper. The secondary piston generates hydraulic pressure in the other half of the brake system.

Because the brake master cylinder is so simple, it is exceptionally reliable. The main concern is leaks. External leaks are not always obvious because the rear seal is buried inside the vacuum booster. Symptoms such as a sinking or spongy brake pedal or dragging or pulling brakes could indicate an internal leak, though. In some cases, the brake warning light, check engine light or a warning message might require you to test the brake master cylinder.

How Do You Test a Brake Master Cylinder?

There are two basic tests you can perform to verify proper operation of the brake master cylinder.

1. In the Car: With the brake system bled, pump the brakes a few times and hold. The brake pedal should be firm. If the brake pedal is spongy, this could indicate air remaining in the lines or a mechanical problem, such as a sticking brake caliper slider. Bleed the brake system again and verify the calipers are moving freely.

On the other hand, if the brake pedal slowly drops, this could indicate a leak. If the pedal drops more suddenly, this is a sure sign that pressure is escaping backwards through one of the internal seals.

Brake fluid leaks at the seals (highlighted in green) are easily overlooked.

2. On the Bench: You can perform a similar test when inspecting a new or remanufactured unit or after removing the existing unit. Start with a bench bleed to remove all air from the master cylinder, then remove the bench bleed fittings and block the ports with bolts — do not over-torque the bolts. Use a screwdriver to press and hold the plunger in the rear of the master cylinder. The plunger should be very firm, if not immovable, past a few millimeters. If the plunger keeps moving in, this indicates a fault of at least one of the internal seals.

Fix Brake Fluid Leaks Before They Cause an Accident

Brake fluid leaks of any kind will put you and others in danger, so any master cylinder fault is reason for replacement. Some brake master cylinders require transferring the reservoir from the old unit. To install this, first bench bleed and check for internal and external leaks. After installation, fully bleed the brake system, making sure the brake fluid does not fall below the low mark. There may be special procedures for vehicles equipped with electric parking brakes, electronic stability control or anti-lock braking.

Check out all the brake system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on brake master cylinders, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.

Benjamin Hunting View All

Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time.  I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

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