Brake fluid is one of the most overlooked maintenance items. It is hygroscopic – it absorbs moisture from the air over time, which lowers its boiling point and causes internal corrosion of the brake hydraulic system. Replacing the brake fluid every 2 to 3 years is essential maintenance that most owners skip until something fails.
Why Brake Fluid Degrades
DOT 3, DOT 4, and DOT 5.1 brake fluids (the common types in modern vehicles) are glycol-based and absorb moisture through the rubber brake hoses, the fluid reservoir cap seal, and any system breathers. After 2 years, brake fluid typically contains 2 to 3% water. The water lowers the boiling point dramatically. Hard braking from highway speed can cause water-contaminated fluid to boil locally near the brake callipers, creating vapour pockets that compress and result in a soft pedal or even brake failure.
How to Test Brake Fluid
An electronic brake fluid tester (a small probe inserted into the reservoir) measures moisture content directly. Test strips that change colour based on copper content (which leaches from internal brake system metals as the corrosion inhibitors deplete) are another option. Visual inspection is unreliable – even very contaminated brake fluid can still look clear. The most practical approach for most DIYers is simply replacing it on a 2-year schedule regardless of testing.
Replacement Procedure
Use the same brake fluid type as currently in the system – DOT 3, 4, and 5.1 are intermixable in an emergency but it is best practice to stick with what is specified for your vehicle. Never use DOT 5 (silicone) in a system designed for DOT 3/4/5.1, or vice versa – DOT 5 is not compatible. Your workshop manual specifies the correct fluid type.
Use a turkey baster or fluid extractor to remove as much old fluid as possible from the master cylinder reservoir. Refill with fresh fluid. Then bleed each brake calliper in sequence (typically right rear, left rear, right front, left front – check your workshop manual for the specific sequence). At each calliper, attach a clear hose to the bleeder valve, route it into a catch container, open the bleeder, have an assistant slowly press the brake pedal, close the bleeder before they release the pedal, and repeat until clean fluid comes out. See our brake bleeding guide for the complete procedure.
Important Cautions
Never let the master cylinder reservoir run dry during the procedure – this introduces air into the master cylinder and you will need to bench bleed it. Brake fluid damages paint quickly – cover any painted surfaces and wipe up spills immediately. Used brake fluid should be disposed of responsibly through a recycling centre, never poured down a drain.
Brake fluid type, capacity, and bleeding sequences are vehicle-specific. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
— MechanicMate . Questions or a second opinion? [email protected].