I had to do some digging through my Road & Track collection, because I
recall a Technical Correspondence anecdote on this very topic. I finally
found it in the November 1989 issue. I'll take a few minutes here to type
this in for everyone to read:
*******
Question:
I have two older Alfa-Romeos with Girling/Dunlop brake components. I
recently drained and cleaned the systems and replaced the old DOT 3 brake
fluid with DOT 5 silicone fluid.
Now I read (In an Alfa club newsletter) that silicone fluids "are known to
be harmful to the rubber seals in the system; they may cause the seals to
swell, or make internal surfaces so slick the seals no longer work..." The
article also says that silicones are "absolutely incompatible with
conventional brake fluids."
Will the silicone fluid damage the old-style rubber seals in my system?
What if there were some DOT 3 fluid left in the system somewhere? What is
the result of the "incompatibility" of silicone fluid with conventional
fluid?
Answer:
A number of readers have written with questions regarding the newer
silicone brake fluid versus the long-established glycol-based kinds. To
sum it up, silicone fluid's main advantage is its inherently noncorrosive
nature. And as any of you who have spilled brake fluid know, glycol-based
fluids can be highly corrosive.
There seems to be a multitude of reasons to stay with the type of brake
fluid originally recommended for the car.
Silicone and glycol fluids are entirely different, and this is where the
first problem arises. Silicones have about three times the amount of
dissolved air as glycol fluids (approximately 15-percent air in silicone
versus 5 percent in glycol). Because of the compressible nature of air,
brake systems using almost all their pedal travel with glycol fluids may
suddenly be inadequate with the silicone fluids, bottoming out before
sufficient system pressure is reached.
The second problem comes from the three different types of rubber used in
the brake system. There is EPDM rubber, SBR rubber and neoprene. Neoprene
is only used for brake hoses, while the seals in the system may be composed
of the two other types. Glycol fluid affects all three types of rubber
uniformly, causing a swell rate of 5-15 percent. This swelling action is
necessary for the seals to withstand the immense pressures to which they
are subjected. Silicone doesn't cause rubber to swell, which is why a
rubber-swell additive is incorporated into silicone fluid. Unfortunately,
current additives are compatible only with the newer EPDM rubber. SBR
rubber, when subjected to silicone fluids, will swell too much, becoming
too soft for adequate sealing.
We are told that most modern brake systems use some EPDM seals, although
more than 90 percent of drum brakes still use SBR seals. Furthermore, most
rubber brake hose is now composed of EPDM rubber.
Now what happens if the fluids are mixed? Remember the rubber-swell
additive used with silicone? Glycol fluids will cause the swell additive
to separate from the silicone fluid. Additionally, silicone fluid will
cause the anti-corrosion additives used in glycol fluids to separate. In
other words, both fluids will lose the additives that allow them to work.
The moral of the story, it would seem, is to leave well enough alone.
*******
So, that's it. I've never SOAKED rubber brake parts in fluid prior to
assembly myself, but I do always coat them with fluid while assembling,
mostly for lubrication. But partially due to this article, I have
refrained from switching to silicone fluid in my TR6.
Pete Chadwell
1973 TR6
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