When should I replace my car's brake fluid?

Once upon a time cars used cable-operated braking systems to come to a halt. And in the future, electric systems will likely be the means of stopping a car.

But for the present, vehicle braking systems are heavily reliant on hydraulic brake fluid.

A hydraulic system pumps brake fluid through a channel, conveying energy from one point to another. In the case of braking systems, the pressure on the brake pedal from the driver's foot pumps brake fluid from the master cylinder in a line to a brake caliper or brake cylinder.

This hydraulic pressure squeezes together the two halves of a brake caliper or expands brake shoes to make contact with a rotating drum fixed to the wheel.

In the case of disc brakes, the caliper clamps the brake pads to a rotor (disc) attached to each of the car's four wheels. This clamping pressure on the rotor slows the rate of wheel rotation and brings the car to a stop.

Brake cylinders (the master cylinder on the firewall and slave cylinders at the wheels) channel hydraulic pressure to the vehicle's drum brakes, if fitted. The pressure in the slave cylinder forces a piston to push brake shoes outwards, increasing the friction between the shoes and the drum fastened to the car's wheel and hub – and thus slowing the wheel's rate of rotation to bring the car to a stop.

Over the years brake fluid has been a 'catch-all' term for castor oil, alcohol, glycol ether (more recently in combination with borate ester), mineral oil and silicon. All of these boil at much higher levels than water – a minimum of 140° Celsius. That's important in any system that builds up heat rapidly when converting kinetic energy (movement) to heat from friction.

Brake fluid should not boil in normal operating conditions, it should not compress and it should not corrode the internal components of the braking system.

As a rule, manufacturers recommend no more than two years apart for the brake fluid to be flushed, replenished and the braking system bled of air bubbles that may have entered the system while refilling. Brake fluid can draw in water through seals in the system, and its properties (high boiling point most importantly) will consequently deteriorate over time.

One obvious indication of a problem is a mushy feel when applying pressure to the brake pedal. Ignore that and brake failure is the next sure sign of neglect, followed shortly by a life-threatening impact with an immobile object.