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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.

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

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.

Jforce wrote:I was told you cant use 5 in mx-3's, its to agressive and eats away the rubber components.

Maybe if you have stainles steel brake lines it won't be a problem.

fowljesse wrote:Good to know. Thanks. I have wondered, but not enough to look into it.

MrMazda92 wrote:My ABS is disabled, but I have yet to completely remove it...

Why is the dot5 fluid not recommended for ABS equipped vehicles? If it's more of a "wear and tear" issue for the ABS, then I couldn't care less. If it's something more serious than that, I'll need to go with the dot5.1 it sounds like.

Thank you for putting this up here, it answered a few of my questions, and gave me new ones!

Another point of confusion is that most people assume that since DOT 5.1 has a higher
number it must therefore be better. This is simply not true. DOT 5.1 was formulated for use
in ABS systems that require a less viscous fluid. From the information I was able to gather it
seems that most road racers are now relying on the latest generation of DOT 4 brake fluid,
along with regular fluid replacement, for the ultimate in hydraulic brake system performance.
So we may conclude that DOT 5.1 is not an improvement, but a modification for ABS use

Ryan wrote:I highly recommend NOT using dot 5 in any vehicle that did not come OE DOT5. You can't just mix them like that, and they react differently with the metals and rubbers in the system. There is a good chance you will die if you put DOT 5 in your MX.