Hi Pete,
I'll throw my .02 into the ring. Converting to silicone in my Spitfire has
been generally
positive. I first tried flushing out the DOT4, and adding the DOT5 and that
was not
satisfactory. The main problem I had was constant seepage from the MC and
wheel
cylinders. I rebuilt the brake system had no further problems. If anyone is
considering
converting to DOT5, I would recommend changing only after a complete rebuild.
A couple of comments about the R&T article:
> 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.
I have not noticed any change in pedal feel or travel after CAREFULLY bleeding
the system. It is easy to introduce small bubbles when pouring the fluid into
the MC reservoir, and I try to avoid moving the brake pedal too quickly when
actually doing the bleeding. I've found that if I bleed the system, then bleed
it
again the next day, a satisfactory pedal feel can be obtained.
> 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.
This may have been true in 1989, and it is well accepted that early formulation
of
DOT5 had problems with the swelling agents, but these problems have been solved.
See <http://www.team.net/sol/tech/dot5.html> for a reference on this point.
> The moral of the story, it would seem, is to leave well enough alone.
Or, if you decide to convert, do it the right way, and for the right reasons.
I think DOT5 has its place, and a weekend driver (like my Spitfire is) is
probably
a good place. When put into a clean system, DOT5 is not corrosive, and won't
absorb water. (Note that this doesn't mean water won't get into the system.
If water
does get into the system, it will not mix with the fluid, and you will get
puddles
of water in the brake lines which could promote local corrosion. Regular
bleeding
will prevent this). Plus, it doesn't damage paint. For these reasons, I have
chosen
to use it. I have not converted my Stag, and I'm not really in a big hurry to
do it
because the braking system is working fine (I don't want to "fix" something that
isn't broken!). If (probably when) I have to rebuild the brake system, I will
probably
convert to DOT5.
Mike Burdick
Durham, NC
'70 Spitfire w/DOT5
'72 Stag w/DOT4
'64 TR4 w/DOT20yearoldgrunge in the brake lines
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