Part explained
Brake fluid change: interval, DOT 4 / 5.1, safety
This is one of the few maintenance jobs where we as editors are unambiguous: **leave this to a workshop unless you really know what you're doing**. Brake fluid is safety-critical — air in the brake system means your brake pedal sinks to the floor on the first emergency stop without slowing the car. This page explains why a change every two years is necessary, which type suits your car, and how the MOT water-content criterion works.
By the onderdelen.autos editorial team · Updated · About the editorial team
What does brake fluid do?
Brake fluid is a hydraulic medium that translates pedal force into clamping force at the brake caliper pistons. It should not compress, must have a high boiling point and must not attack the rubber parts in the system. The problem: brake fluid is hygroscopic — it absorbs water, even through rubber hoses and small vent openings in the reservoir cap. After two years, average brake fluid contains 2 – 3% water. That water boils at around 100 °C; during hard braking on a descent or sporty driving, the fluid near the calipers easily reaches that temperature, and steam bubbles form that let the pedal sink.
Symptoms — often none until it's too late
- Softer pedal feel than before, especially after hard braking on a descent.
- Pedal slowly sinks when held down at standstill.
- MOT measurement: water content above 3% = advisory or failure.
- Dark or brown fluid in the reservoir (fresh is light amber).
- Sudden pedal sink during hard braking — stop immediately, do not continue driving.
When to change
Indicative every **2 years**, regardless of mileage. The Dutch MOT (APK) has measured brake fluid water content since 2020; above 3% you get an advisory or fail. Some manufacturers spec 3 years or 60,000 km — go with whichever is sooner. When in doubt: have a workshop measure the water content (€ 5 – € 10) before deciding. **This isn't a job to postpone** — the boiling point of contaminated brake fluid drops well below factory spec after four years.
Typical interval: 30,000–60,000 km
What does it cost?
The fluid itself is cheap: € 10 – € 25 per litre for DOT 4 or DOT 5.1. An average car needs 0.5 – 1 litre. The real cost is workshop service: € 40 – € 100 for a complete change including bleeding all four calipers. A vacuum bleeder does the job in 20 minutes; manual bleeding with a second person takes an hour. Expect € 50 – € 125 total at a workshop — and that's money well spent.
| Condition | From | To |
|---|---|---|
| New | € 10 | € 25 |
Compare DOT 4 and DOT 5.1 brake fluid from Dutch sellers and supply it yourself to your workshop.
DIY? Honestly: usually no.
Difficulty:
**Read this carefully.** Changing brake fluid is technically not a complex operation, but the consequences of a mistake are fatal. An air bubble left in the system, a bleed nipple that doesn't seal, or the wrong bleed order — on the first emergency stop your pedal sinks to the floor. No vacuum bleeder, no workshop manual for the bleed order, or no experience? **Have a workshop do it.** This isn't a job where you learn by trying. An € 80 workshop invoice is cheaper than a collision. Also important: never mix DOT 4 with DOT 5 (silicone-based — not compatible). DOT 5.1 is compatible with DOT 4 and has a higher boiling point.
- Vacuum bleeder or a bleed kit with hose and catch bottle
- Fresh brake fluid in an unopened bottle (open contact with air contaminates it immediately)
- A second person to operate the pedal during manual bleeding
- Clean rags — brake fluid attacks paint aggressively
- Knowledge of the correct bleed order for your car (in the workshop manual)
Frequently asked questions
What type of brake fluid is in my car?
DOT 4 is standard for virtually all modern passenger cars. DOT 5.1 is an upgrade with a higher boiling point — compatible with DOT 4. The type is printed on the reservoir cap. **Never** use DOT 5 — that's silicone-based and absolutely not compatible.
What's the difference between DOT 4 and DOT 5.1?
Both are glycol-based and miscible. DOT 5.1 has a higher boiling point (260 °C dry vs 230 °C dry for DOT 4) and is interesting for heavy use (mountain driving, towing, sporty driving). For normal city use, DOT 4 is fine.
How do I know it's time to change?
First check: age. Older than 2 years = change. Second check: colour in the reservoir. Fresh fluid is clear light amber; dark brown or black = overdue. At MOT you get a water-content measurement; above 3% is a direct trigger.
What happens if I don't change it?
Water content rises, boiling point drops. Under normal city driving you often notice nothing — until the day you take a long descent or make an emergency stop. Then the water in the calipers boils, steam bubbles form, and the pedal sinks without braking effect. This isn't theory, it shows up on Dutch MOT statistics every year.
Can I buy used brake fluid?
No. Sealed, unopened bottles only — the moment brake fluid contacts air it absorbs water. A half-empty bottle from a previous job is unusable within weeks.
Related parts
Brake fluid from Dutch inventory
Order sealed bottles of DOT 4 or DOT 5.1 from Dutch sellers — often with bundled service from affiliated workshops.