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[bmw] Re: Oops Brake Fluid...

As others have already pointed out, DOT3 and DOT4 fluids are compatible. However, the DOT3 fluid will reduce the boiling point of the combination to something between DOT3 and DOT4 values, depending on the ratio of DOT3 to DOT4.

Given that you merely topped off the reservoir with DOT3 brake fluid, and that the brake system does not circulate fluid, you still have DOT4 fluid through the master cylinder and, more importantly, in the brake lines and calipers. So, rest easy knowing that you haven't compromised the brake system or any components.

If you plan to do any driving school events this year, you should replace the fluid at the time your car is tech inspected. Otherwise, don't be worried about it, and replace the fluid next year.

-rb

Ron Buchalski
BMWCCA #76387
1995 E36 M3
1993 E34 525iT
1999 Mazda Miata



Gerry Phillips wrote:

>I messed up and I need to know how badly.  I checked my brake fluid
reservoir on my '89 535i this past weekend and it was low.   I've topped
off my brake fluid in the past and I know my car calls for Dot 4, but for
some reason I picked up Dot 3 and topped it off with that.  I guess I was
too busy thinking about spending a nice Valentine's evening with my
fiancee.  I realized it after I had already done it.  My questions are:
Will Dot 3 hurt anything?  Should I have the system drained and refilled?
And what are the differences between Dot 3 and Dot 4?
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