Hydraulic Clutch Oil??

[AZRickD]

Okay...

Let's say somebody (like me), just put in DOT3 in his '05 EC250 (DOT4 Only inscribed on the reservoir cap).

What's the prognosis? If I have to purge the DOT3 out of the system, how do I go about doing that. Should I get a rebuild kit to replace the seals?

Gawd I love google and the internets...

Here's a good article from bimmerroadfly.com

http://bimmer.roadfly.com/bmw/forums/e39/8995265-1.html

DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3 (401F), and both fluids will exhibit a reduced boiling point as water content increases. DOT 5 in its pure state offers a higher boiling point (500F) however if water got into the system, and a big globule found its way into a caliper, the water would start to boil at 212F causing a vapor lock condition [possible brake failure -ed.]. By contrast, DOT 3 fluid with 3% water content would still exhibit a boiling point of 300F. Silicone fluids also exhibit a 3 times greater propensity to dissolve air and other gasses which can lead to a "spongy pedal" and reduced braking at high altitudes.

DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of such a mix being a lowered boiling point. In an emergency, it'll do. Silicone fluid will not mix, but will float on top. From a lubricity standpoint, neither fluids are outstanding, though silicones will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. Since few of us ride at temperatures very much below freezing, let alone at 40 below zero, silicone's low temperature advantage won't be apparent. Neither fluids will reduce stopping distances.

From stoptech.com:

As a trailing note on the DOT ratings, if your car was designed for a particular type of fluid (especially prior to the development of DOT 4 fluids), you should make every attempt to stick with that fluid! For example, if your car was delivered with DOT 3 fluid, the internal components of the system (seals, brake hoses, and fittings for example) were specifically designed and tested for compatibility with DOT 3. Because DOT 4 fluids contain a different chemical composition, the system may not necessarily react in a positive fashion to the borate esters floating around in the mix.

In other cases, just the difference in viscosity of the two different fluids may cause the seals to wear at different rates. What starts as an annoying squeak might eventually become a torn seal or worse. The examples could go on and on, but the message here is this: it’s fine to upgrade from DOT 3 fluid A to DOT 3 fluid B, but you should think twice (maybe even three times) before switching from DOT 3 fluid A to DOT 4 fluid of any sort.

That said, when dealing with modern hydraulic braking systems a numerically higher DOT rating is typically considered to be compatible with a lower DOT rating (except for DOT 5, of course). Unfortunately, this same generality just isn’t true for most older hydraulic system materials.

Something I found on TSX Club.com:

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.

From stoptech.com:

As Carroll accurately pointed out, DOT 3 fluids are usually glycol ether based, but that is not because they are required to be. In fact, FMVSS116 makes no mention whatsoever about the chemical compounding of brake fluids – it simply dictates the fluid physical properties. However, the brake fluid industry has by consensus decreed that glycol ether fluids are the most economical way to meet the requirements, so there you are.

These glycol ether fluids are typically a by-product of the process used to make certain paints and varnishes. By definition, DOT 3 fluids must have a minimum dry boiling point (measured with 0% water by volume) of 401F and a minimum wet boiling point (measured with 3.7% water by volume) of 284F. That’s really about all the specification says as far as the performance enthusiast is concerned.

DOT 4 fluids are also glycol ether based, but have a measure of borate esters thrown in for improved properties including increased dry and wet boiling points. A seldom talked about characteristic though is that because of this chemistry, the DOT 4 fluid will have a more stable and higher boiling point during the early portion of its life, but ironically once the fluid does actually begin to absorb water its boiling point will typically fall off more rapidly than a typical DOT 3. By FMVSS116 standards, DOT 4 fluids must have a minimum dry boiling point of 446F and a minimum wet boiling point of 311F.

Does this make DOT 4 fluids better than DOT 3 fluids? Not always. Remember, the boiling points listed are minimums and there are DOT 3 fluids out there with higher boiling points than some DOT 4 fluids. The real differentiating factor should be that if you run a DOT 4 fluid you really should change the fluid more often than if you use a DOT 3, if for no other reason than the rapid fall off in boiling point with time.

We won’t even discuss DOT 5 fluids as they are completely unacceptable to the high-performance enthusiast, but we’ll include them in the following table for completeness.