In a message dated 11/9/09 2:23:23 PM, healeys-request@autox.team.net
writes:
> You should never fill up with DOT5 if you previously used DOT3 or
> >> 4. You run the very serious risk that the seals will perish and the
> >> system will leak like a sieve.
>
Sheesh! it always amazes me when these discussions rear their heads again.
The key difference between (unrestored) 100s and later cars is that the
early cars used real rubber in the seals and hoses, and later cars used a
synthetic mix of rubber and other stuff. All brake fluid is formulated to
cause
the seals to swell a bit so that they will, well, seal. Silicone fluid uses
different additives than glycol fluid, and the additives are not compatible,
cancelling one another out, hence the cautions not to mix them, and to flush
out the glycol if and when switching to silicone. The additives in silicone
don't work on natural rubber, and in fact will cause natural rubber to
degrade.
So, bottom lines (repeating these for the umpteenth time):
If you wish to use silicone fluid in a system that still has its original
seals, replace the seals with new ones.
If you wish to use silicone in a system that has had glycol in it, flush
out the system with e.g. brake cleaner or alcohol.
Silicone has the advantages of greater longevity, is not damaging to paint,
and is good in cars that are driven infrequently, so every classic car
museum of which I'm aware uses silicone fluid in its cars.
Glycol has the advantage of providing a firmer pedal, but must be replaced
more often, and is very damaging to paint.
Glycol and silicone can not be mixed without losing the integrity of the
braking system.
Even silicone fluid should be bled out and replaced every few years.
Cheers
Gary
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