Question 1

When brake fluid needs changing

Accepted Answer

Brake fluid is hygroscopic — it absorbs moisture from the air. Fresh fluid has a boiling point around 260 °C, but after two years the moisture content rises to 3–4 %, dropping the boiling point below 160 °C. Under hard braking — a long downhill, or behind a heavy vehicle — temperatures easily exceed this, the fluid boils, and the brake pedal sinks straight to the floor.

Manufacturers recommend a fluid change every 1–2 years or every 30 000–40 000 km. Many cars' service schedules require it to preserve warranty. Moisture content can also be measured in seconds with an electronic tester — the check is always free with any service.

Beyond moisture, the fluid collects metal particles from the calipers and master cylinder. Old fluid is brown or black, whereas fresh fluid is clear or slightly yellow.

Question 2

How the change is done

Accepted Answer

The change is performed with a pressure bleeder that pushes clean fluid into the master cylinder reservoir under constant pressure. Each caliper is opened in turn — rear right, rear left, front right, front left — and old fluid is discharged via a bleed hose.

The process continues until only clear new fluid, free of bubbles, emerges at every caliper. On ABS-equipped cars the diagnostic tool cycles the ABS pump valves individually to make sure old fluid is also flushed from the unit.

The reservoir is filled to the correct level, the cap is resealed, and the pedal is checked for firmness and feel. A test drive confirms even, reassuring braking performance.

Question 3

Why does the brake pedal feel spongy?

Accepted Answer

A spongy or occasionally bottoming pedal almost always signals air in the hydraulic circuit or fluid that has gone too old. Air can enter through an over-low reservoir level, or when a caliper has been opened without proper bleeding. Aged fluid saturated with moisture starts to vaporise under heavy use — the pedal feels normal for the first couple of stops but sinks to the floor on the third. Diagnosing the cause requires pressure-testing the circuit and checking the fluid's moisture content, which tells us whether a fluid change is enough or whether the master cylinder or hoses need replacing too.

Brake Fluid Change in Vantaa