Understanding Brake fluid
Glycol based brake fluids...
DOT 3
DOT 4
DOT 5.1
Silicone based brake fluids...
DOT 5
WARNING: Dot 5.1 glycol (yellow) and Dot 5 silicone (purple) will not mix.
Glycol based fluids are designed to signal moisture contamination by
changing color... the fluid will start to turn golden, then light
brown, then dark brown indicating that it has absorbed progressively
more moisture. Eventually, if left unchanged beyond the recommended
service interval, the fluid will become dark and yukky, indicating
high amounts of water absorption and thus badly contaminated fluid...
User friendly Glycol based fluids also reduce the effect of both
corrosion and compressibility because it is not only designed to
accept significant amounts of moisture, but even to neutralize it by
dispersing this moisture evenly throughout the system, thus preventing
its concentration in any one area..
Glycol based fluids
(NOTE) DOT 5 Silicone based fluid will not turn color to signal
moisture content...
Glycol dry boiling point versus Glycol wet boiling point 3% moisture...
DOT 3 205 °C (401 °F)...............140 °C (284 °F)
DOT 4 230 °C (446 °F)...............155 °C (311 °F)
DOT 5.1 260 °C (500 °F)............180 °C (356 °F)
Silicone dry boiling point versus Silicone wet boiling point 3% moisture...
DOT 5.1 270 °C (518 °F)..............190 °C (374 °F)
If you wish to use the latest in Glycol base fluids then its DOT 5.1
Note: DOT 5 silicone was created at the request of the military for
use in vehicles that are required to be parked for years at a time and
be ready for deployment. Harley-Davidson used DOT 5 until a decade ago
because it would not negatively affect paint but currently specifies
DOT 4 or 5.1
WARNING: Dot 5.1 glycol (yellow) and Dot 5 silicone (purple) will not mix.
TRACK
As a rule always install fresh brake fluid before a race or track day because you may
boil the tiny percent of moisture in brake fluid and end up with no brakes... I speak
from experience on the RC45...