What Is A Brake Master Cylinder And How Does It Work?

What exactly is a brake master cylinder in a car? How does it even work and how much is the replacement cost? Here's a definitive guide.

The brake master cylinder is an integral part of all modern cars.

It controls your braking, converting the depression of a pedal to stopping force. Impressive.

As you might imagine, the brake master cylinder is crucial. Severe problems with it might manifest in an unavoidable high-speed crash.

That’s the last thing anyone wants, so as soon as you notice any issues with your brakes, go to your local mechanic!

Key Takeaways

  • The brake master cylinder consists of two spring-sat pistons. These apply pressure to the brake fluid, which squeezes the brakes and slows the wheels.
  • Brake master cylinders are usually pretty reliable.
  • The most likely failure point on brake master cylinders is the rubber seals.
  • If your brake master cylinder fails, check for listed recalls.
  • Don’t drive on a failing brake master cylinder!

Table of ContentsShow

What Does A Brake Master Cylinder Do?

brake master cylinder
A brake master cylinder.

A brake master cylinder is responsible for taking the force from the pedal and transferring it to the calipers at the wheels.

In other words, it uses push force to apply hydraulic pressure to the brake lines. These are filled with hygroscopic brake fluid.

The fluid pushes against the piston in the wheel brake, slowing your car down.

All cars should have a split brake system (the oldest classics might not). It should be a diagonal split these days, although older vehicles might have a front/rear system.

I’ll explain a little more about split braking systems below.

Master cylinders are used in all sorts of hydraulic systems in various applications. Brakes are just one example.

How Does A Brake Master Cylinder Work?

A brake master cylinder is a fairly basic component in automotive terms. However, it might be challenging to get your head around without a visual aid, so I’ve included the video above.

A master cylinder contains two internal, in-line pistons. The brake fluid reservoir sits on top, with the brake lines feeding out from specific points.

Car brake master cylinders function as follows:

  1. Before pressing the pedal, brake fluid sits in the space past the pistons. This is due to small holes in the top of the master cylinder/bottom of the reservoir.
  2. The pedal is pressed, pushing a piston forward. The force is magnified by the brake booster (vacuum servo).
  3. The brake booster pushes the pistons, compressing them against the springs they sit on.
  4. The brake fluid sitting in front of them is pushed down the circuits.
  5. The calipers (discs) or wheel cylinders (drums) apply frictional material to the brake. This slows the wheel’s rotational movement, reducing your car’s speed.
  6. The pressure releases when you lift the pedal, with all the fluid returning to its ‘rest’ position.

What Is A Split Braking System?

Car Suspension and Brakes

Your car technically has two braking systems controlled by the master cylinder simultaneously. Under normal circumstances, they both operate, and you never have to think twice about it.

Split systems are used to ensure you can still bring your car to a stop if one brake line fails. You’ll still have brakes on two out of four wheels. Stopping distance will increase, but you’ll be able to gently bring your vehicle to a halt.

The most common type of split system is a diagonal split. Here, your front-left and rear-right brakes are on the same circuit. The same is true of the front-right and rear-left.

Certain vehicles feature a split between the front and rear. In such instances, the pair of front wheels operate on one hydraulic circuit, as do the pair of rear wheels.

Check out the diagram below.

Split Braking System Types
Image source: Auto Chimps.

Diagonal splits are almost always used these days. That’s because they’re much safer. You still get stopping power at the front wheel (the most significant) and some assistance from the opposite rear one.

There’s a potential danger in a front/rear split system. If the front line splits and fails, you’ll only have the rear brakes. Since all the weight (engine, transmission, etc.) and braking momentum (dive) is at the front, rear brakes alone are almost useless. They would bring you to a stop, but very slowly.

You won’t find right/left split systems. If one circuit were to fail, this system would cause you to spin under heavy braking. The car’s rear would kick out to the side with the leaking brake line.

Is The Brake Master Cylinder Important?

The brake master cylinder is absolutely vital. As a component in the hydraulic braking system, not having one means the brakes won’t work. Like, at all.

Notice any symptoms of it failing?

Don’t ignore it!

You might end up in a situation that could be dangerous to your life if it worsens.

If it’s damaging, you’ll detect numerous very clear indications (see below).

What Makes A Brake Master Cylinder Fail?

brake master cylinder

Brake master cylinders are usually quite dependable. In various instances, they will endure as long as the vehicle.

Certain vehicles will be more prone to master cylinder failure. You might find recalls for certain cars and trucks.

Also, if you do a lot of city driving, you use the brakes much more often than a highway commuter. By inference, there’s more wear and strain on the master cylinder, so it’s more likely to stop working.

I should also point out that your driving style is a factor. Smooth braking is much better (emergencies aside, of course).

Slamming the brake on by stamping on the pedal means the master cylinder experiences a much more sudden force. Doing this multiple times daily means you’re more likely to see failure in the future.

How Do You Know If The Brake Master Cylinder Is Bad?

NOTE

Brake problems are almost always rooted in the brake lines or brakes themselves. The master cylinder could be the root problem, but it’s less likely.

If the brake master cylinder fails, it’s almost certainly leaking. In other words, one of the rubber seals has gone brittle and cracked.

You’ll notice the following symptoms:

  • Low brake fluid – it’s leaking out.
  • Dirty brake fluid – contaminants from the air and engine are getting in.
  • Spongey brake pedal – there’s air in the system.
  • Dashboard warning lights – most relatively modern cars will notify you of brake problems.

Can I Replace My Own Brake Master Cylinder?

man checking car brake master cylinder

I highly recommend not doing it unless you possess a solid mechanical comprehension. Fixing the brakes demands some expertise and carefulness.

For example, if you forget to bleed the brakes, you won’t be able to stop.

In other words, yes – it’s possible. But don’t risk it. Leave it to the experts who have all the necessary tools.

How Much Does A Replacement Brake Master Cylinder Cost?

Dollars (Price, Cost)

As I mentioned above, in the unlikely scenario that you have a bad brake master cylinder, the issue probably lies with the seals.

Listen to your mechanic’s advice, but you shouldn’t need an entirely new unit.

Instead, they’ll remove the component and change the seals. They’ll then reinstall it and bleed the brakes, topping up the hydraulic fluid.

Seal replacement should be around $250.

If you require a new master cylinder, prepare to spend approximately $600 (including service).

Driving With A Broken Brake Master Cylinder: Should You?

Negative.

It’s like driving after removing at least two of your brakes. It doesn’t make sense.

If the car ahead of you slams its brakes on, or there’s an emergency of some kind, you won’t be able to stop. Insurance liability may decide it could have been avoided if you had a working brake master cylinder.

Why take the risk? If you clearly have no working brake master cylinder, call a tow truck and head to your local mechanic.

You Might Love These

disc brakes with brake caliper
Got Grinding Brakes? Here’s How You Fix It
Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!