When To Change Your Brake Fluid with Chart

Palm Abbe for Brake Fluid

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
High Temp

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
DOT4

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.