Symptoms of Master Cylinder Failure

Depressing the brake pedal

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Of all systems on your vehicle, the brake system could be the most important. When the driver steps on the brake pedal, a brake booster amplifies the force, pushing directly into the master cylinder. The master cylinder converts linear motion and force into hydraulic pressure. The “master” cylinder distributes this pressure to the brake calipers or wheel cylinders, also known as “slave” cylinders. At the slave cylinders, hydraulic pressure is converted back to linear motion and force, to compress brake pads or expand brake shoes. In turn, the friction generated can keep a vehicle from moving, or slow it down, converting its kinetic energy into heat energy.

Here, we discuss how the master cylinder works and what symptoms are associated with master cylinder failure. Some of this information may not apply to some newer brake systems, which feature integrated electrohydraulic boosting, but the theory is similar.

How Does the Master Cylinder Work?

A diagram of a typical brake master cylinder

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Before learning how the master cylinder might fail and how to recognize problems, it’s good to understand how it works. On top of the master cylinder is the brake fluid reservoir, usually attached directly, but sometimes connected by a hose. Gravity feeds brake fluid to the master cylinder, filling the space around two pistons, one for each circuit. At rest, return springs push the pistons to the back of the master cylinder, releasing all pressure from the brake lines.

When the driver depresses the brake pedal, the brake pedal pushrod pushes on the primary piston. As the primary piston moves forward, it moves past the intake port and generates hydraulic pressure, which is directed to the primary brake circuit and the secondary piston. Because brake fluid doesn’t compress, the secondary piston moves forward at the same time, generating hydraulic pressure in the secondary brake circuit. Depending on brake system design, primary and secondary circuits may vary, usually front (primary) and rear (secondary), but some vehicles split the hydraulic system diagonally or some other way.

Symptoms of Master Cylinder Failure

A brake warning light

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Like all mechanical and hydraulic devices, the master cylinder will eventually wear out. Depending on use, the typical master cylinder might last 60,000 to 200,000 miles. Highway commuters use the brakes less often than city taxis, for example, so their master cylinders tend to last longer. The mechanical parts of the master cylinder, the springs, and pistons are so simple that failure is almost unheard of. On the other hand, the rubber seals can wear out and degrade over time, leading to internal or external leaks. Here are a few symptoms of master cylinder failure, along with some basic brake diagnostic tips.

  • Sinking Pedal: When the driver steps on the brake, it should depress to a certain point and stay there. If the pedal continues to drop, it could indicate an internal brake fluid leak. Before condemning the master cylinder, check brake fluid level and check for external leaks from brake lines, calipers, or wheel cylinders. 
  • Contaminated Brake Fluid: Over time, rubber seals can degrade and disintegrate, resulting in “dirty” brake fluid. This can be caused by incompatible or hydrated brake fluid. To help these master cylinder seals last longer, always use the fluid specified in the Owner’s Manual, fill from a sealed container, and replace the brake fluid every 20,000 miles.
  • Brake Warning Light: This warning light can come on for different reasons, depending on the design of the vehicle. Usually, the brake warning light will illuminate to indicate low brake fluid level or parking brake engagement, and some vehicles may also illuminate the brake warning light to indicate brake pressure problems. Before condemning the master cylinder, though, check for leaks in other parts of the brake system.
  • Check Engine Light: Because the master cylinder is critical to other systems, like anti-lock brakes (ABS) and electronic stability control (ESC), master cylinder pressure problems may cause these systems to record diagnostic trouble codes (DTC) and possibly disable auxiliary systems.
  • Low Brake Fluid: While leaks at brake lines, brake hoses, wheel cylinders, and brake calipers commonly result in low brake fluid, they are usually easier to identify because they are more exposed. External master cylinder leaks can be hard to identify because the only leak point is at the rear, which is often hidden in the brake booster. If the rear seal leaks, it will often leak directly into the booster, though you may be able to see brake fluid residue at the seam between the master cylinder and the booster. Remove the master cylinder to check more closely for leaks.

Basic Master Cylinder Repair

A typical brake master cylinder

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For the most part, problems with the master cylinder are solved by replacing the master cylinder entirely. True, they can be rebuilt, but such a critical component is best left to the professionals. Some new or rebuilt master cylinders may not come with the reservoir, so the old one will need to be cleaned and installed on the new one. Master cylinder bench-bleeding and installation tends to be messy, so be sure to cover painted surfaces and clean up everything as soon as you can get all the lines attached and before the reservoir runs out.