PS Magazine - TB 43-PS-632

PS, The Preventative Maintenance Monthly

ISSUE 632

JULY 2005
PS Magazine - TB 43-PS-632 - Page 14 of 33
PS 632
JUL 05
24
BFS + Unknown Fluid
Try mixing a few tablespoons of
the unknown fluid with a little
BFS. If the two mix, the
unknown fluid is BFS, too. But
if the two fluids separate into
layers, the unknown stuff is DOT
3 or 4. Your vehicle needs to
have the brake fluid changed.
Brakes…
BFS
,
provided under MIL-PRF-46176B and also known as DOT 5 brake fluid,
replaced the old brake fluid, V-V-680, also known as DOT 3 or DOT 4. BFS is more
stable at high temperatures and won’t absorb water.
So, when it’s time to add brake
fluid to a master cylinder containing
BFS, make sure that what you add is
also BFS. A combination of BFS
with old fluids will still absorb water,
creating corrosion and deposits that
cause stopping problems.
Mixing old brake fluid with BFS
can also lead to seal leakage, because
the seal protective properties of BFS
are diluted by the old stuff.
So how do you tell which fluid is which? Go by the color, if you can. BFS is pur-
ple or blue in color, though the dye that gives it the color can break down. Then the
fluid in the master cylinder becomes brown or amber.
There’s no cause
for alarm if the
color
varies—the
BFS is still good.
But if the color is
not purple or blue,
you can’t tell what
kind of fluid is in
your brake system.
…especially
when the
temperature hits
triple digits.
Unknown Fluid + Water
Put some of the unknown fluid
in a jar with a little water and
shake it. BFS does not mix with
water, and you’ll see distinct lay-
ers. DOT 3 and 4 fluids, on the
other hand, do mix with water
and remain mixed. You won’t
see separate layers.
dot 3 or 4
fluids should
be
flushed!
here
are two
other
ways to
tell what
kind of
brake
fluid you
have…
see your
tm for
details.
BFS is
purple
or blue
Check
brake
fluid for
color
Mix?
Both BFS
No mix?
DOT 3 or 4
Mix?
DOT 3 or 4
No mix?
BFS
Keep this info
about silicone
brake fluid
(BFS) in mind…
632.24-25(C)
5/27/05
9:45 AM
Page 1
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