One of the many Scotts on this list discusses DOT5 brake fluid, and
in considering switching over asks:
The disadvantage is that I have to get rid of all the old DOT-4 already
in there or the new stuff will be "impaired".
Thus my questions:
- What does "impaired" mean in this context? How will I know if I
haven't properly flushed the system?
Basically the DOT-[3,4] and DOT-5 fluids do not mix. Any remaining 4 fluid
bubbles in the system may eventually collect with other bubbles, forming
larger plugs of DOT-4 in the lines. Now, since 4 will absorb water and 5
won't, guess where any water vapor introduced into the system will gather?
Yes, the plugs of 4 will become more and more saturated over time. This
leads to a couple of things happening.
First, if the plugs tend to stay in one general area of a line, that area will
be more prone to corrosion. Secondly, since the dilution of DOT-4 fluid by
as little as 5% water can reduce the boiling point by nearly half, these soggy
puppys can cause trouble if they are in calipers or cylinders. It won't take
much heat to boil them, producing steam, which is quite a bit more compressible
than brake fluid. Not a good thing for good pedal feel.
Personally I've not considered switching to silicone fluids, as I feel that
Castrol GT LMA fluid is the best DOT-4 fluid generally available, with a dry
boiling point about 85% of the better silicone fluids. Of course, I have yet
to own a car where I really worry about the paint. If and when I get the TR6
or the 3 or even the 250 repainted, I may be reconsidering my choice of
fluids.
mjb.
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