DOT3 vs DOT4 vs DOT5 - Brake Fluids Explained

[Noel]

What are the differences and what do they mean? Much perhaps a topic I never come across on Honda forums. I'm going to give you a simple idea on what each category of brake fluids mean and which is the one most suitable/applicable for our CL7/9s for road and track work. After all, selecting the correct and most appropriate brake fluid comes hand in hand with how good your brakes work under all situations and circumstances.

Polyglycol Based DOT3 - Color - Amber | One of the most common finds on the market and have a specific for cars designed for the late 80s period. Although compatible with our CL7/9s should you cannot find the required selected grade, this is not the correct specification to be used from factory recommendation guidelines. DOT3 also absorbs moisture the quickest and has the lowest boiling point and should only strictly be for road use.

Polyglycol Based DOT4 - Color - Amber | The benchmark specifications by the majority of car manufacturers, this is also the specific grade to be used with Hondas/Acura cars regardless of models. DOT4 is designed to absorb moisture slower than DOT3 and has a higher boiling point. This is both suitable for road use and track work. While the OEM Honda DOT4 brake fluid is sufficient for weekend trackdays, meets etc. Using racing specific DOT4 refined brake fluids will increase higher temperature tolerance particularly under hard braking conditions.

Silicone Based DOT5 - Color - Purple | Least common and also the most different from both grades above. DOT5 are primarily designed for cars not equipped with ABS units (Unless otherwise manufacturer specific)and this should not be used unless you're absolutely sure of what you're doing. DOT5 does not absorb moisture and holds the highest boiling point. This brake fluid is not compatible with either categories above and should never be mixed due to it's chemical properties.

Always take care when handling brake fluid as this will corrode body paint work in a matter of seconds if contact is made. Should accidental spills or drips happen, immediately flush affected surface area with high spraying water and wipe off area.

Should you choose to switch to a higher category or quality/grade brake fluid, it is strongly recommended that you perform a complete brake fluid flush. Never use a mineral or synthetic based oil ala engine oil or transmission fluid in substitute of specific designed brake fluid. Such incompatibility will cause all rubber components in the brake hydraulic system to swell up and leak.

[1TSXer]

you forgot about DOT 5.1 which can be used/mixed with DOT3 & 4.

[StopTech]

Here is some more about brake fluid
http://www.stoptech.com/whitepapers/brakefluid1a.htm

[CZ-75]

Someone had a "decay" chart of the BPs of leading performance brake fluids after 6 months, a year, etc. as they had adsorbed moisture. Anyone know about this?

[moda_way]

This is from Motul's white papers comparing 5.1 to the others

The high wet boiling point (185°C / 365°F), superior to DOT 4 (155°C / 311°F mini) and DOT 3 (140°C / 284°F mini) fluids enables to use this product longer. Indeed, DOT 3 / DOT 4 and DOT 5.1 brake fluids have the property to absorb humidity contained in the air, which reduces their boiling points and consequently security.
The wet boiling point is representative of the fluid after one year of use.

Specially designed to anti-locking brake systems (ABS) :
The viscosity (820 centipoises at -40°C / -40°F) lower than DOT 4 (up to 1800 cp) and DOT 3 (up to 1500 cp) brake fluids, allows an easier fluid circulation in micro-valves of anti-locking systems.