What Does A Brake Servo/Brake Booster Actually Do?

The brake servo is one of those car components most of us are aware exists, without necessarily knowing exactly what it does or how important a role it plays in stopping your car.

Well, in short, it’s an important step between you pushing the brake pedal and the brakes being applied, and the thing that stops the pedal from taking vast amounts of effort to operate.

Image via Wikimedia Commons/Ildar Sagdejev

As a reminder, pressing your brake pedal forces a pushrod into the sealed master cylinder, which is filled with oil. A pair of pistons displace the oil, with then travels down the brake lines to the brake calipers. Hey presto, your caliper’s pistons are actuated, pushing the pads against the disc.

The addition of a servo into the equation isn’t always vital. In fact, some cars – many racing cars, for instance, as well as the McLaren F1 – don’t have one. What a servo does is dramatically reduce the physical effort needed to actuate your brakes with your foot – cars with only a master cylinder require a lot more pressure for significant braking force to be applied.

Image via Wikimedia Commons/Paul Day

A vacuum brake servo is the most common type. This vacuum is created inside the main body of the servo via a pipe running to the engine’s air intake – that’s why your brake pedal feels different with the car switched off. In diesel-powered cars, a hydraulic pump – driven by the engine – is added to generate the vacuum.

Once the pedal is pressed, a rod is pushed into a housing which contains two springs and an air filter. Air begins to flood one side of the servo body, which is separated into two sections via a diaphragm. A vacuum remains in the side connected to the air intake or hydraulic pump, creating a pressure difference between the two chambers.

An early form of vacuum brake servo, dating from 1928

This pressure difference causes the diaphragm to be pulled towards the master cylinder, forcing the pushrod into it with the help of a spring. Thus, your brakes are applied with a little helping hand. Without all this happening, you’re relying on the strength of your right foot alone to actuate the brakes – which isn’t good news if you’ve skipped leg day.

If you notice you’re having to apply more pressure to get your brakes working than before, it could be a sign that your servo is in need of attention. Meanwhile, it is possible to fit a better servo to your car, but this is very uncommon as far as brake system upgrades go.

This Caterham’s master cylinder does not have a servo

Just remember the next time you hit the brakes – there’s even more going on behind the scenes than you might have realised.