Brake fluid is easy to forget because it doesn't get used up the way oil or fuel does. But it does wear out, and old fluid is one of the most common reasons for a soft pedal or brake fade.
What brake fluid does
When you press the pedal, the master cylinder pushes fluid through the lines to the calipers (or wheel cylinders on some older cars). Fluid doesn't compress, so the force goes directly to the pads. For that to work, the fluid has to stay liquid and free of air, even when the brakes are very hot.
Why it goes bad
Most common brake fluids (DOT 3, DOT 4 and DOT 5.1) are glycol-based and hygroscopic, which means they absorb moisture from the air over time. Water in the fluid:
- Lowers the boiling point. Under hard braking, the water can turn to vapor, which compresses and leads to a soft or sinking pedal.
- Promotes corrosion inside calipers, lines and the ABS unit.
Fluid that was clear and amber when new often turns darker as it ages. Color is a rough hint, but moisture test strips or a shop's moisture tester are more reliable.
DOT ratings in brief
- DOT 3, DOT 4 and DOT 5.1 are glycol-based and can generally be mixed, though mixing lowers performance to the weaker fluid. Always use the type specified for your car.
- DOT 5 is silicone-based. It doesn't absorb water the same way, but it should not be mixed with glycol fluids. It's sometimes used in classic cars that sit a lot, usually after a complete system rebuild. It is not a drop-in replacement for modern ABS-equipped cars.
- High-temperature "racing" fluids have higher boiling points but often need to be changed more often.
Check your owner's manual or the cap on your master cylinder reservoir for the fluid your car requires.
When to change it
Follow the interval in your owner's manual. Many owners also flush fluid:
- Before and after track days
- When the pedal feels softer than it used to
- When buying a used car with no record of a brake fluid service
Flushing vs. bleeding
- Bleeding removes air from the system.
- Flushing replaces all the old fluid with new fluid.
On ABS-equipped Corvettes, some procedures require a scan tool to cycle the ABS pump. Check your factory service manual before you begin.
Safety note
Brake fluid damages paint, so cover nearby painted surfaces and wipe up spills right away. If your pedal feels soft or goes to the floor, don't drive the car until the cause has been found and fixed.