Here is my basic rant about DOT4 vs DOT5 - it's from a racing/high
performance slant, not concours or preservation.
If you don't want to read through it all, there's a summary at the end.
An even terser summary: no on in their right mind races with DOT5 fluid.
----
First thing to know is that there are two ways to categorize brake fluid: DOT
3/4/5 and glycol vs. silicone.
The DOT numbers represent a set of specifications, the key components of which
are as follows:
DOT level DOT3 DOT4 DOT5
Dry boiling point, min. 205C (400F) 230C (450F) 260C (500F)
Wet boiling point, min. 140C (280F) 155C (310F) 180C (350F)
Viscosity at -40degC 1500 1800 900
(centistokes)
The thing to realize is that, strictly speaking, the DOT spec number does not
specify which chemical composition must be used for each level of spec,
although the requirements for each level for temperature properties and
chemical compatibility with different braking system components pretty well
dictates what goes on. The most important meaning of the DOT numbers has to do
with the wet and dry boiling point of the fluid, and its viscosity when
extremely cold (as it would be nice for the brakes to continue to work when the
temperature falls to -40degC).
Now, typically, the DOT5 fluids have only been implemented with silicone
fluids. This is changing: Castrol SRF is apparently a DOT 5 fluid that is not
made with silicone; they apparently hold the patent and are selling the
formulation exclusively under their name. I have heard rumors of other
non-silicone fluids being available in Europe. DOT 5 silicon fluid does not
adsorb (adsorb should be the correct word - not absorb) much water, unlike DOT
3 or 4 (which has a max of ~5-6% adsorbtion, if I remeber correctly). Water is
not miscible in the silicone fluids, whereas the glycol/polyglycol fluids
(which are usually the DOT3s) and the borate esters (usually the DOT4s) are
very hygroscopic.
According to Car Design and Technology Magazine (the Dec. 1991 issue,
from which most of this material is taken), the biggest source of
water contamination for the fluid is osmosis through the rubber flex
lines- which explains why the fluid in the calipers is always nastier
than the fluid in the reservior, and why calipers corrode into a state
of uselessness long before master cylinders.
Additionally, the DOT4 fluids absorb water more quickly than the older
DOT3s, but the effects on the boiling point are more severe for the
DOT3s. In either case, the fluid's boiling point is reduced by from
50-70C (120-160F) within 12 months for the DOT4s, and 80-100C
(170-210F) for the DOT3s.
Remember that the DOT ratings are only a *minimum* spec. So, Ford Heavy Duty is
a DOT-3 fluid with an _Outstanding_ dry boiling point comparable to that of
AP550 racing brake fluid. it is rated DOT-3 because its wet boiling point is
mediocre; since racers are supposed to change their fluid out regularly anyway,
we may infer that Ford Heavy Duty is an appropriate fluid for the track, and so
it is.
(If you look at the numbers in the table above and think "Oh, I should switch
to a silicone-based DOT5 fluid", don't. If there is sufficient interest, I will
go into the details as I know them, but suffice it to say that silicone fluids
are not compatible with existing fluids, requiring a complete flush and clean
with alcohol, and are terribly hard to bleed well, leaving that uncomfortable
squishy feeling.)
|