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Brake fluid - DOT 3 / DOT 4 ?


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Hi everybody,

did some maintenance work today on my 96' SLS. I had planned a brake fluid change and got 5 litres of DOT 4 brake fluid. Could not get any DOT 3 fluid, shop said DOT 4 superseded DOT 3. The can also says will replace DOT 3.

I did the flushing with the DOT 4 (non synthetic) and it worked fine so far.

Is there any reason I should be concerned about the use of the DOT 4 fluid? I read that the DOt 4 has a higher boiling point, but that's beneficial...are there other differences?

Best regards


1996 SLS German spec

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Switching to DOT 4 is not a good idea. I would recommend another complete brake fluid flush and get back to a DOT 3A or DOT 3B fluid.

DOT ratings

What exactly is the DOT rating telling us? More importantly, what is the DOT rating NOT telling us? A quick look at FMVSS116 – the US Government’s Specification for brake fluids – will tell us all we need to know…

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.

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.

DOT 4 fluids are also glycol ether based, but have a measure of borate esters thrown in for increased immunity to water absorption. Because of this chemistry, the DOT 4 fluid will have a more stable 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. See attached image

As a trailing note on the DOT ratings, if your car was designed for a particular type of fluid, 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 certain 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.



Drive'em like you own 'em. - ....................04 DTS............................


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George, Bigfoo,

thanks for your replies. Now I'm worried... :huh:

I planned a visit to my Cadillac dealer on monday anyway to get coolant pellets, so I'll ask them whether they have DOT 3 fluid. As the dealer is in the biggest city around, there are also many auto part shops I can check!

I'll let you know what comes out.

Thanks again



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George, that was some great reading on Dot3 and Dot 4. I thoroughly enjoyed your post. Thanks for the detailed information.


"Burns" rubber

" I've never considered myself to be all that conservative, but it seems the more liberal some people get the more conservative I become. "

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Hi all,

I found some DOT 3 brake fluid today and exchanged the fluid again. It should be fine now.

I also went to our area's main (and only) Cadillac dealer to get some of the coolant pellets, guess what they said? "Sir, you've got a northstar engine with factory dexcool filling, you don't need this supplement." When I asked them to sell me some anyway, they couldn't find any in the store, they don't stock it any more! By chance they found one leftover package which I could purchase. So I could perform my coolant change today too.

Thanks for your support,


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