When To Change Your Brake Fluid with Chart

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.

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