The fluid in the reservoir was judged to be contaminated — usually dark, cloudy or visibly full of debris. Brake fluid absorbs moisture from the air over time, and moisture lowers its boiling point, which is what causes a long pedal after heavy braking.
Wording as it commonly appears on an MOT certificate: “Brake fluid contaminated”.
What does it mean?
The fluid in the reservoir was judged to be contaminated — usually dark, cloudy or visibly full of debris. Brake fluid absorbs moisture from the air over time, and moisture lowers its boiling point, which is what causes a long pedal after heavy braking.
How serious is it?
Usually recorded as: Major. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- Fluid that's simply never been changed — most manufacturers specify roughly every two years
- A perished reservoir cap seal letting damp in
- Rubber debris from a deteriorating master cylinder or caliper seal
- The wrong fluid type having been topped up at some point
What a mechanic would check
A moisture test on the fluid and a look at its colour against fresh fluid — new DOT4 is nearly clear, and if what's in the reservoir looks like weak tea it's overdue. I'd also check the level and look for any sign of where fluid has gone if it's low, because a dropping level means a leak somewhere.
What might need replacing
- A full brake fluid change and bleed
- Reservoir cap seal
- Investigation of a leak if the level was also low
Can you still drive it?
Failure item. Beyond the MOT, contaminated fluid is one of those faults you don't notice until you need the brakes hard twice in a row and the pedal goes long. It's a cheap job with an outsized safety benefit.
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This is general guidance from a UK mechanic, not a substitute for a physical inspection. Only the tester who inspected your vehicle can confirm what was recorded and why.