MOT defect file
Brake fluid contaminated or boiling point reduced
Last updated
Quick answer
The brake fluid has absorbed water or picked up contamination, which lowers the temperature at which it boils. Boiled fluid turns to vapour, and vapour compresses, so the pedal goes soft exactly when you need it most.
- Typical cost
- £60–£130
- DIY viability
- Possible with care
What you might notice
- The pedal goes long and soft after repeated heavy braking, then feels normal again once everything has cooled
- Fluid in the reservoir that has darkened to the colour of weak tea instead of pale straw
- Brakes that feel fine on a normal commute but fade alarmingly on a long descent or after towing
- No day-to-day symptom whatsoever on a car that is never braked hard, which is why contamination usually goes unnoticed for years
What causes it
- Age. Brake fluid is hygroscopic — it pulls moisture out of the air through the reservoir breather and the hoses, whatever you do
- Missing the manufacturer's fluid change interval, which is typically every two years regardless of mileage
- The wrong fluid or a mineral-based oil added by mistake, which swells every rubber seal in the system
- A reservoir cap left off or a perished cap seal, letting damp air in directly
Advisory or failure?
Fluid that is contaminated, or whose boiling point has been seriously reduced, is a major defect and a failure. Where the tester judges that brake failure is imminent as a result, it is graded dangerous. In practice this is assessed by looking at the fluid and, in garages that have one, with a fluid test strip or electronic tester rather than by any measurement built into the MOT itself, so it is a judgement call in a way that pipe corrosion is not. Fluid that is merely a little darker than new, with no evidence of contamination, would not normally be recorded as a defect — but it is a fair reason for the tester to advise a change.
Can you fix it yourself?
Possible with care
A fluid change is a two-person job done the traditional way: one pumps the pedal, the other opens and closes each bleed nipple in the right order, usually starting at the wheel furthest from the master cylinder. Buy a sealed bottle of the correct specification and use it straight away, because an opened bottle starts absorbing moisture on the shelf. The traps are worth knowing. Bleed nipples seize and shear off, which converts a fluid change into a calliper replacement, so release them carefully and use a proper bleed spanner. Never let the reservoir run dry mid-job. Cars with ABS often need the module cycled with a diagnostic tool to flush the fluid trapped inside it, and on those a garage flush is the more honest option.
What it costs
This is one of the few brake jobs where labour dominates almost completely: a litre of DOT 4 is £8 to £15, and everything else is the hour it takes. A conventional flush and bleed on a car without complications runs £60 to £90. Cars with ABS or an electronic parking brake that need the module cycled or the brake put into service mode sit at the top of the band, because the job needs a diagnostic tool plugged in. Add cost if a bleed nipple shears — that is when the price jumps, since replacing the calliper it was fitted to is the only remedy.
What happens if you ignore it
Water in the fluid boils at a far lower temperature than the fluid itself. Under sustained braking — a long hill, a heavy load, a spirited drive — that water turns to vapour, and because vapour compresses, the pedal sinks to the floor and the brakes simply stop working until it condenses again. It is the classic cause of complete brake fade in a car whose pads and discs are perfectly good. Moisture also corrodes the system from the inside out, pitting calliper bores and master cylinder seals, so what would have been a £70 flush becomes a £400 calliper and seal job. And because the fault is invisible at the roadside, most people discover it at the worst possible moment.
Common questions
- Is contaminated brake fluid an MOT failure?
- Yes. Fluid that is contaminated, or whose boiling point has been seriously reduced, is a major defect and a failure, and it is graded dangerous where the tester judges brake failure to be imminent. Fluid that is merely a shade darker than new, with no evidence of contamination, would not normally be recorded, though a tester may advise a change.
- How often should brake fluid be changed?
- Follow the manufacturer's interval, which is typically every two years regardless of mileage. Brake fluid is hygroscopic, meaning it draws moisture out of the air through the reservoir breather and the hoses whether the car is driven or not, so a low-mileage car needs the change just as much as a high-mileage one.
- Why does my brake pedal go soft on a long hill?
- Water absorbed into the fluid boils at a far lower temperature than the fluid itself. Under sustained braking it turns to vapour, and vapour compresses where fluid does not, so the pedal sinks towards the floor and the brakes stop working until it condenses again. It is the classic cause of fade on perfectly good pads and discs.
- How can I tell if my brake fluid has absorbed water?
- Colour is the rough guide: fresh fluid is pale straw, and fluid darkened to the shade of weak tea is well past its best. Colour alone is not proof, though. A garage can measure moisture content properly with a test strip or an electronic tester, which is what the MOT judgement usually rests on, since nothing is built into the test itself.
- How much does a brake fluid change cost?
- A litre of DOT 4 is £8 to £15 and the rest is the hour it takes, so a conventional flush and bleed runs £60 to £90. Cars with ABS or an electronic parking brake that need the module cycled sit at the top of the band. The price jumps if a bleed nipple shears off, because that calliper then has to be replaced.
The warning light behind this
This defect is what a tester writes down. What you see from the driver’s seat is a symbol, and it is worth checking which one, because manufacturers reuse the same shape for more than one system.
This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.
Has this been recorded on your car?
Check the full MOT history for any UK registration.