Brake fluid is hygroscopic — it absorbs moisture from the air through the master cylinder seals. Wet fluid boils under heavy braking and corrodes ABS modulators internally.
The real schedule
Every 2 years regardless of mileage. DOT 4 absorbs ~3% water per year in UK climate — at 6%+ water content, boiling point halves.
Symptoms of old fluid
Soft pedal after a hill descent, ABS warning light, MOT failure on fluid moisture test (DOT4 >3% water = fail).
ABS module damage
Wet brake fluid corrodes the internal ABS valves. A new ABS module is £600–£1,500. A brake fluid change is £60–£90. Maths is easy.
Mobile service
Full pressure bleed at your driveway, all 4 corners + ABS module bleed where supported. 45-minute job.