DOT DOT DOT
by Jim Lunson
got a surprise the other day when I went to my trusty auto parts store
for some brake fluid to flush out my lines. I looked for my
stalwart, Castrol DOT 4 with LMA and found it is no longer made.
It is now synthetic and new formula. This change led me to
some questions where I found out that it is really the same stuff and
easily mixed with the old. But this started me on some research on
brake fluids to see what is going on here.
Brake fluid is
designed to be as close to an inert fluid as possible. The goal is
for it to do virtually nothing, not boil, not freeze, not expand, not
evaporate, not anything; just sit in the system and move the brake pads
and shoes when called for. The problem is that this type fluid is
almost impossible to make. The main restriction found in the fluid
today is in the boiling point, and this is where the Department of
Transportation (DOT) comes into play.
Brake fluid gets very hot as it works the disc brake
calipers and drum shoes. The brakes work on friction which
generates heat, and the piston or cylinder is right next to the pads and
gets very hot as the brakes are applied (just touch the wheels after a
long run with lots of downhill braking required). The higher the
boiling point, the better the fluid will perform. As the fluid
reaches the boiling point under severe use, it vaporizes into a gas
which is compressible. This results in the brakes fading, becoming
spongy, and not able to stop the car. This is not good.
I ran into this recently on my summer trip to
Colorado. Ann and I drove up the road to the summit of Pikes Peak.
This road is quite a climb, going up over 5,000 feet in less than 20
miles. On the return trip down, we kept seeing signs to use
lowest gear, hot brakes will fail, and about halfway down, we
passed a checkpoint where a forest ranger stops each car and points a
temperature sensing gun at the front brake caliper. There is a big
sign that says any brakes reading over 350° must stop here for 30
minutes before proceeding further. My brakes registered 300o so I
was able to continue. There is, conveniently, a gift shop at this
spot, but since he let me go on down, it is a real concern and not just
a gimmick. But the brakes get that hot and can fail due to boiling
of the fluid.
Because of this safety issue in stopping the car, DOT
got involved, evaluated brake fluids and rated them according to their
boiling temperature (STD #116) with the following table:
DOT 3 311°
DOT 4 356°
DOT 5 376° (the odd numbers are conversions from centigrade temperatures used by DOT)
This is the only criteria for their rating. It
has nothing to do with the material of the fluid, only the boiling
point. The problem and misconception comes in that DOT 3 & 4
fluids are alcohol/glycol-based and work reasonably well in most
applications, although their boiling point is fairly low.
Silicone-based fluids have a higher boiling point, and they qualify as
DOT 5. But it is only the performance, not the material, that
yields the rating. Lockheed now makes a glycol-based fluid which
meets the DOT 5 criteria, but has been given the designation DOT 5.1
to avoid confusion because it is glycol based rather than silicone.
And you can see from my experience on Pikes Peak, these temperatures
mean something in braking the car.
But, back to my original dilemma, it seems my Castrol
DOT 4 fluid has always been synthetic, they just decided to add the new
buzz word on the label. And although it may be a new formula, it
is still only DOT 4 rated, is still alcohol/glycol based and is
completely compatible with their old product. So don't panic at
the new product, it is the same old stuff and can be added to the DOT 4
fluid already in the system.
There is an issue in mixing fluid types. The
problem is moisture. DOT 3, 4, and now 5.1 type fluids are
alcohol/glycol-based and this material absorbs water. As moisture
from the air touches the system (such as when you check the master
cylinder fluid level), it enters into glycol-based systems. Once
in the system, it slowly causes corrosion and rusting as it touches the
metal parts. The longer this fluid remains in the system, the more
moisture it takes on, and the faster the deterioration occurs.
That is why it is important to replace the fluid every few years to
maintain the system in good condition. I have found the Castrol,
with their LMA additive (low moisture additive) helps prolong the life
of the fluid (but not indefinitely) a few more years while the DOT 4
boiling rating helps over DOT 3 in braking performance, especially on
long downhill runs. DOT 5 is silicone based and does not mix with
either water or the glycol fluid.
fluids do not have this moisture absorption problem as it will not take
on water. Silicone fluids, therefore, will last a great deal
longer, are easier on the rubber gaskets and seals, and with the higher
boiling point, provide the best braking system available. That is
why the US military specifies only silicone fluid for all its vehicles.
Here maintenance is usually suspect and to avoid any confusion, it is
all they use.
Mixing two types of fluids in the brake system will
not work. That would mean two distinct non-mixing fluids
flowing in the same lines, causing all sorts of problems. Flow
will be restricted, moisture will concentrate in low spots, the
expansion due to heat and boiling will vary and braking will be
adversely affected. So dont mix the two types under any
circumstances. Silicone will not absorb water, but it will not
remove any moisture already in the system either. Water will just
sit in the low spots causing severe rust. The only way to use
silicone is to completely clean all parts of the system of any moisture
or glycol. This includes the master cylinder, wheel cylinders and
pistons, as well as all lines and hoses. Any moisture remaining in
the system will cause the same rust and corrosion wherever it remains.
This is why switching to silicone is best done only when a complete
brake system replacement is done, and everything is spotless.
One final note: brake fluid is not considered a top
off type fluid. The brake system is sealed and the fluid does not
evaporate so if the level in the master cylinder drops, it is the sign
of leak problems somewhere in the system and should be checked
thoroughly. The only reason for fluid level going down is as the
pads wear the pistons and cylinders move slightly outward, but this
decrease in fluid level is very slight and occurs over the life of the
brake pads, not weekly or monthly. So, watch your fluid level, and
if it needs topping off frequently, there is a leak somewhere in the
system and it should be attended to soon. And if you have to add
fluid, make sure you do not mix the two types of fluids.