Yousuf Khan said:
Can you mix them? How do you know which one is in there currently?
Yes, you can mix DOT 3 & DOT 4 (see the label on the bottle). You
can't mix either of these with DOT 5, which is silicone-based and
completely incompatible.
Not all DOT 3 & 4 brakes fluids are created equal. Some are much
better quality than others. DOT 3 & 4 simply set minimum standards
that the fluids must meet. Things to look at on the label are the
minimum wet boiling point temp (the higher the better). This is the
temp at which brake fluid that has absorbed the maximum amount of
water from the air begins to boil. You may also see a minimum dry
boiling point temp, which is always higher than the wet b.p., and
represents the temp at which fresh, unadulterated fluid begins to
boil. Not nearly as useful a figure.
It's always best to flush out as much of the old fluid as possible
by bleeding all four wheels in the proper sequence. If you don't
know how, or if you have an ABS brake system which may be
more difficult to work on, then leave it to a professional.
Bob