DOT 3 / 4 or DOT 5? What Brake Fluid to Use

How You Know It's Right
A properly bled brake system will yield a firm, consistent brake pedal feel. To double-check for a properly bled system simply look to the master cylinder. Remove the cover while an assistant pumps the pedal several times, holding it down on the final pump. Observe brake fluid squirts within the master cylinder as the pedal is quickly released. The fluid should squirt no higher than 3 inches from the fluid's surface. A well-bled system is sometimes the only difference between your car stopping behind the car in front of you or underneath the car in front of you. Improperly bled systems have also led to the false assumption that some monster-sized rotor and multi-piston caliper set is needed when, in reality, a $5 bottle of fluid will often do the trick.

No Refills!
Just because your brake master cylinder reservoir is low doesn't necessarily mean you should top it off with fresh fluid. As brake pads wear, pistons move outward in order to maintain an equal distance between the pads and rotor. As such, as pads wear, the brake fluid level drops in order to compensate for the increased displacement. If the pads are known to be in new condition, it's time to look for a leak.

Which Dot Is Right For Me?
There are four different types of DOT-certified brake fluids to choose from, however, the higher number fluid isn't always what you need:

DOT 3: This is the most common fluid. DOT 3 fluid absorbs approximately 2 percent moisture per year, which isn't a good thing by the way since moisture absorbed through microscopic seams and cracks can corrode the system and thicken the fluid. This is partially why brake system bleeding is recommended every 30,000 miles. DOT 3 has a dry boiling point of 401 degrees F and a wet boiling point of 284 degrees F.

DOT 4: Although twice as expensive, DOT 4 fluid absorbs far less moisture than DOT 3, has a dry boiling point of 446 degrees F, and a wet boiling point of 311 degrees F. DOT 4 and DOT 3 fluids can be mixed together.

DOT 5: Silicone-based brake fluids like DOT 5 do not absorb moisture at all and, as such, can't be mixed with DOT 3 or DOT 4 fluids. When switching to DOT 5 it's important to flush the entire system at least a few times. DOT 5 fluids exhibit much higher boiling points but are not compatible with most Hondas since they aren't ABS friendly. DOT 5 fluids easily absorb air bubbles that can negatively effect ABS systems and braking in general. Few Hondas should be using DOT 5 fluids.

DOT 5.1: Unlike DOT 5, this fluid is non-silicone-based and has a boiling point over 500 degrees F and can be mixed with DOT 3 and DOT 4 fluids. These three fluids are all clear in color, while DOT 5 fluid is purple.