How, when, and where you drive all affect when you need to change your brake fluid. If the vehicle is used for towing, driving in mountainous regions, driving at high speeds, or has ABS brakes, it is best to change the fluid when the refractometer reading shows 2% water content or less. All vehicles should have the brake fluid changed when the water content exceeds 3%.
The below chart shows what refractometer readings mean for 4 common types of brake fluids: DOT3, DOT 3 High Temp, DOT 4, and DOT 4 Super.
Condition | Water Content | Boiling Point (F) | Boiling Point (C) | |
DOT3 | OK | Less Than 2% | Above 356 | Above 180 |
Borderline | 2% to 3% | 320 to 356 | 160 to 180 | |
Change | Greater than 3% | Less than 320 | Less than 160 | |
DOT3 | OK | Less Than 2% | Above 374 | Above 190 |
Borderline | 2% to 3% | 320 to 374 | 160 to 190 | |
Change | Greater than 3% | Less than 320 | Less than 160 | |
DOT4 | OK | Less Than 2% | Above 410 | Above 210 |
Borderline | 2% to 3% | 369 to 410 | 187 to 210 | |
Change | Greater than 3% | Less than 320 | Less than 187 | |
Super | OK | Less Than 2% | Above 414 | Above 212 |
Borderline | 2% to 3% | 374 to 414 | 190 to 212 | |
Change | Greater than 3% | Less than 374 | Less than 190 |
Many motor vehicle companies recommend changing the brake fluid every 12 months, while others recommend changing it every 24 months. The amount of moisture that brake fluid absorbs depends on a number of factors including the type of fluid, humidity, type of brake hoses, condition of the seals, and the vehicle’s mileage.
Generally, brake fluid will absorb at a rate of 1% or more moisture per year of service life. A two-year-old car will have 2-3% water in the brake fluid. 2% water reduces the boiling point of DOT3 brake fluid by approximately 135 degrees Fahrenheit (75 degrees Celsius). 2% water in DOT4 brake fluid reduces boiling point by 81 degrees Fahrenheit (45 degrees Celsius).
MISCO has a selection of refractometers programmed specifically for measuring brake fluids: here.