Marc,
Apparently DOT5 (silicone) fluid has a quite different appearance - purple
(I think) instead of the light brown of normal brake fluid. If there's any
fluid left in the M/C, or if you can pump a little out of one caliper, that
should tell you.
I have heard that DOT4 is a bad idea because the mineral oil eats the
natural rubber seals, not sure how much truth their is in that or not.
Probably best to stick with DOT3 to be on the safe side, if you use DOT3 or
4. These can be safely mixed I believe.
If you switch to DOT5 silicone fluid (NOT the same as the DOT5.1 stuff you
sometimes see, not sure what that is), many believe that this needs to be
done in conjunction with a rebuild of all your brake parts, replacing every
bit of rubber in the system, or the rubber may perish. There are some who
say that they have done just fine by flushing all the old fluid out of the
system using 'mineral spirits' (I guess that may be American for White
Spirit??), and filling with silicone fluid, without replacing anything.
Also, Silicone fluid is significantly more expensive than normal brake
fluid! It does have its advantages - spills won't eat through your
paintwork, it will not absorb water which leads to corrosion of brake pipes
(unless they've been replaced with copper), it needs bleeding less often.
As for brake feel, opinions differ - I think Silicone is less compressible,
which means brakes will come on more suddenly, and the brake will move less
between the slightly-applied position and full-on position - some prefer
this, some don't!
One word of warning - be gentle when undoing your bleed nipples!! If they
won't budge easily, spray with penetrating fluid and leave for a while.
Apparently a hammer tap on the end helps as well - best use a small bit of
wood between the hammer and the nipple to avoid damage to the nipple. I
currently have THREE broken half-nipples stuck in my brakes, and am awaiting
replacement calipers and rear cylinders, having failed to get the old bits
out. Not good, and not cheap.
Richard & Daffy
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