The article is MOSTLY accurate, but still perpetuates some of the most
common misunderstandings about DOT 5 and silicone brake fluids.
To put the rest of this in perspective, we have one MG running DOT 5
(converted 3 years ago as part of a total system rebuild of both brake
and clutch hydraulics), and another running DOT 4 (not had to do the
hydraulics yet). Before changing the one car over, I did a LOT of
reading, because I didn't want to spend that much money without being
absolutely sure that I wanted the results. Both cars go (and stop) just
fine thank you, and both are in daily usage.
> DOT5 brake fluid is also known as "silicone" brake fluid.
Yes, that is the common vernacular, but DOT 5 is a specification for
performance, not composition. For the sake of brevity (and my sanity),
I will use DOT terminology: DOT 5 means fluid that meets DOT 5
specifications, and SBBF means "Silicone Based Brake Fluid".
BTW: The reason that DOT 5.1 is called that (and not something else) is
that, technically, it is a modification of the DOT 5 specification. In
essence, DOT 5.1 meets DOT 5 specs for boiling point, but differs on pH,
chemical stability, and a few other factors.
There is a standard for the composition of SBBF, but that is entirely
separate from the DOT 3,4,5 criteria
> Since DOT5 does not absorb water, any moisture in the hydraulic system
> will "puddle" in one place. This can cause localized corrosion in the
> hydraulics.
If you assume that DOT 5 and SBBF are the same (which they aren't but
this is the last time I will say that), I suppose if you hold your head
just right, this is theoretically true. But in the real world, if SBBF
doesn't absorb water (as the writer notes), and it's put into a clean
system (as the writer recommends), how is the water that's supposed to
be catastrophically "pooling" going to get in there in the first place?
Inquiring minds want to know.
> Careful bleeding is required to get all of the air out of the system.
> Small bubbles can form in the fluid that will form large bubbles over
> time. It may be necessary to do a series of bleeds.
Very true for SBBF. SBBF traps air bubbles easily, and releases them
VERY slowly. Can make it a major pain to bleed unless you are very
patient by nature. The bubbles can be VERY tiny, and don't have to form
large bubbles to cause a problem.
> DOT 5 is slightly compressible (giving a very slightly soft pedal),
I can't believe that these two statements are right together. Brake
fluid (regardless of composition) is a liquid. Liquids don't compress.
Displace, yes, but not compress. That's how hydraulics work to multiply
force. However, AIR will compress, and as noted, it is all too easy to
still have air in a system with SBBF. If you have a soft pedal with
SBBF, bleed it again, c-a-r-e-f-u-l-l-y.
> and has a lower boiling point than DOT4.
No, it doesn't. Quoting from 49 CFR 571.116:
S5.1.1 Equilibrium reflux boiling point (ERBP). When brake fluid is
tested according to S6.1, the ERBP shall not be less than the following
value for the grade indicated:
(a) DOT 3: 205 deg.C. (401 deg.F.).
(b) DOT 4: 230 deg.C. (446 deg.F.).
(c) DOT 5: 260 deg.C. (500 deg.F.).
S5.1.2 Wet ERBP. When brake fluid is tested according to S6.2, the
wet ERBP shall not be less than the following value for the grade
indicated:
(a) DOT 3: 140 deg.C. (284 deg.F.).
(b) DOT 4: 155 deg.C. (311 deg.F.).
(c) DOT 5: 180 deg.C. (356 deg.F.).
And while I'm ranting:
"Silicon" is a mineral, used as an abrasive and as a substrate for
integrated circuits.
"Silicone" is a polymer, used for brake fluid, certain lubricants,
waterproofing and , uh, certain forms of cosmetic surgery.
Okay, I'll get off my soapbox for now...
Chris K.
ckotting@core.com
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