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What’s a Brake Master Cylinder?

A brake master cylinder is a fluid-filled cylinder connected to all four-wheel brakes using brake lines. The master cylinder is mechanically connected to the brake pedal.

Master cylinder

Pressing the foot brake causes the piston fitted inside the master cylinder to pressurize the fluid within the cylinders and actuate all four brakes.

What Does The Master Cylinder Do?


The master cylinder employs two compression chambers or cylinders and is known as a tandem brake master cylinder. The tandem cylinder set-up is a safety feature; if one compression chamber fails to build pressure, the second chamber will still work independently.

As each cylinder is independent and only feeds two wheels, a cylinder failure means a vehicle will still have at least two wheels with effective brakes and those brakes are typically paired diagonally.


Where’s a Brake Master Cylinder Located?

The brake master cylinder is located under the hood at the firewall directly behind the brake pedal assembly (most vehicles). Identifying the brake master cylinder is easy; it’s fitted with two hard brake lines, has an opaque plastic fluid reservoir on top, and is bolted directly to the brake booster, which resembles a large disc shape.

Brake master cylinders are usually bright silver as they are made from rust-resistant alloy.


Master Cylinder is located at the firewall.

Brake Master Cylinder Faults

Brake cylinders aren’t hugely complex, but they are carefully calibrated to work without fault. Although not common, brake master cylinders do simply wear out.

The master cylinder employs a single cylinder to which a piston is fitted. The piston is designed to pressurize two chambers within the cylinder.

To pressurize these chambers, the piston is fitted with seals, and it is these seals that wear out and cause brake pressure loss. While not common, it happens to older classic cars or vehicles with poor service records. Old brake fluid attacks master cylinder seals and moisture within a poorly maintained system will cause corrosion which can damage seals.

Seals may be replaced, but it’s a job for a specialist shop, and a remanufactured unit is a common approach to repairing a faulty one.

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