What Are The Signs Of A Bad Master Cylinder?

A bad brake master cylinder can manifest itself in several ways. 

Here are the most common signs:

1. Misbehaving Brake Pedal

The brake pedal is often a mirror for problems in the brake system. 

With a bad brake master cylinder, your brake pedal will likely feel soft and mushy. There may be excessive brake pedal travel, and it might even sink entirely to the floor. It’ll be slow to bounce back to its original position, or worse, not return at all. 

The common culprit is often a damaged rubber seal inside the master cylinder. 

More importantly, if the master cylinder doesn’t function correctly, the brakes won’t receive enough pressure to efficiently stop the wheels. 

Meaning, your brake calipers won’t get sufficient pressure to clamp the brake pads onto the brake rotors. And wheel cylinders will have trouble delivering brake shoe pressure onto a brake drum.

2. Brake Fluid Leak

A brake fluid leak can happen at almost any point in the braking system — from the master cylinder, brake line, to the caliper piston in disc brakes (or wheel cylinders in drum brakes). 

Fluid leakage from the master cylinder can be due to a few things, like: 

• Damaged master cylinder rubber seals
• Unsecure master cylinder reservoir
• Cracked old master cylinder housing or brake fluid reservoir

So, it’s always a good idea to regularly check the brake fluid reservoir level and keep an eye out for any brake fluid leaks under your car.

3. The Brake Warning Light Is On

If your vehicle has brake fluid or pressure sensors, they’ll set off an alert when the brake fluid level or hydraulic pressure is too low. This is when the brake warning light or check engine light illuminates. 

If the brake warning light pops on, it’s a clear sign that something is wrong with the brake system. 

But the check engine light can imply many things, not just braking system problems. In this case, you’ll likely need an OBD code diagnosis to isolate the actual issue.

4. Contaminated Brake Fluid

The master cylinder seals not only ensure that the brake fluid stays in — they also keep external contaminants like dirt, dust, and rust out. 

Damaged or broken seals won’t only leak fluid. They’ll also let all these contaminants and air in, adversely affecting braking performance. 

New brake fluid is typically a clear, amber tone (if you’re using the usual DOT 3 brake fluid or DOT 4). Contaminated brake fluid can range from dark brown to almost black. 

And while brake fluid will naturally degrade over time, fluid that’s degrading faster than usual, combined with other bad brake symptoms, can indicate brake hardware issues. 

5. Vehicle Drifts When Braking

The brake master cylinder typically has two separate hydraulic brake circuits, where 1 brake circuit controls 2 wheels. This is a safety measure to ensure that the other circuit can still stop the vehicle if one hydraulic circuit fails. 

A failure in one brake circuit may cause the vehicle to drift to one side when the brakes are applied, as only one circuit is functioning effectively. 

6. Uneven Brake Pad Wear

If one of the hydraulic circuits in the brake master cylinder develops an issue, uneven brake pad wear may occur. 

In this case, one set of brake pads will be more worn than the other as the one with hydraulic circuit problems won’t be functioning efficiently. This, again, can translate to vehicle weaving whenever you press the brake pedal. 

How Much Does A Brake Master Cylinder Replacement Cost?

A brake master cylinder replacement typically costs between $402 to $503. 

This breaks down to:

• Labor costs: estimated between $136 to $171
• Parts: estimated between $267 to $332

Keep in mind that these numbers don’t include taxes and fees, and don’t factor in location or specific vehicle make. Also, other necessary repairs can increase the final cost.

How Critical Is A Brake Master Cylinder Replacement?

It’s never safe to drive any vehicle that has braking issues.

A malfunctioning component like the master cylinder could cause a total loss of braking capability. The best option is to get your vehicle off the road and fitted with a replacement master cylinder ASAP.

4 Brake Master Cylinder FAQs

Let’s get to know the brake master cylinder a little bit more:

1. What Is A Brake Master Cylinder?

The brake master cylinder plays a significant role in your car’s braking system. It converts force from your foot on the brake pedal into hydraulic pressure at the braking mechanism, stopping the wheels.

Here’s what happens: 

When you depress the brake pedal, a brake booster amplifies that force and activates a push rod. The push rod plunges a piston through the master cylinder, converting mechanical force into hydraulic pressure. 

This brake pressure is transmitted through each brake circuit to the brakes at the wheel. 

If you have disc brakes, the caliper piston activates a brake pad pair to compress a brake rotor.

If you have drum brakes, wheel cylinders push out brake shoes against a brake drum. 

2. How Long Do Master Cylinders Last?

In a brake master cylinder, the component that probably wears down the fastest is its rubber seals. These regular wear components can typically last between 70,000 to 125,000 miles. 

However, it really depends on your driving conditions and habits. Lots of aggressive braking or frequent exposure to stop-and-go traffic will certainly make the master cylinder work harder. 

So, to keep your brake master cylinder in good working order, it’s best to stick to a proper maintenance schedule. Maintenance can include a brake fluid flush, component inspection and cleaning, and repair or replacement of wear and tear parts. 

3. Is It Hard To Replace A Brake Master Cylinder?

The answer would depend on how comfortable you are dealing with mechanical parts. 

Generally, a brake master cylinder repair isn’t too complicated. However, it does require preparation, and you may need to remove other engine components, hoses, or wires to access the master cylinder. 

So, when in doubt, get an auto repair professional to deal with any brake repair. Your brakes are too important to allow any margin of error.

4. How To Replace A Brake Master Cylinder (A General How-To)

Here’s a general walkthrough for a brake master cylinder replacement:

First, locate the various components of the brake master cylinder: the master cylinder reservoir (brake fluid reservoir), the master cylinder, the brake line connections, and the brake fluid level sensor.

Then:

• Remove the brake fluid from the master cylinder reservoir using a siphon or a turkey baster. 
• Detach the brake fluid sensor connector.
• Loosen the brake line fittings and detach the brake lines.
• Detach the old master cylinder.
• Bench bleed the new master cylinder to expel excess air.
• Install the new master cylinder.
• Reconnect the brake lines and brake fluid sensor connector.
• Top up the master cylinder fluid reservoir with new brake fluid.
• Bleed the brakes to remove any air bubbles from the brake system.

Remember that brake fluid is corrosive and can damage painted surfaces. Keep a brake cleaner handy to help remove any spilled brake fluid.