Brake Fluids Dot 3,4,5 and Silicone

[Mike Wattam <MikeWattam@domain.elided>]

Hi Glenn

Silicon fluid always has DOT 5 specification in the UK, this is based on
it's high specification and thus beats nearly all ester-based fluids
hands-down.

Conventional ester-based brake fluids are highly Hygroscopic, that is they
accept water readily into suspension, and for this reason no brake fluid
container should ever be left open, and tins with rerlatively low fluid
levels should be thrown away as the water in the air within the tin is
enough to make the fluid very bad very quickly.  

This damn hygroscopy can create big problems over a year or two, in that
the water content can migrate straight down the pipes and into the brake
cylinders so that when enough heat builds up, the water will boil and the
pedal will completely disappear.  Which is why the VM's these days say
change the fluid very regularly.  I've only experienced this once, and
NEVER will I again.

The secondary effect is what you have experienced, the internal corrosion
of brake components.

Silicone brake fluid has none of these 'endearing' features, so is highly
recommened.

Silicone brake fluid has also had a bad press about lack of brake 'feel'
and a spongy pedal, but this has never been backed up by any actual
evidence.  Personally if such a thing occurs I would always blame bad
operator failing to bleed the brakes properly.

I have had silicone fluid in my Stag for 10 years now (no change of fluid
or any seals in that time) and no sticking rear wheel cylinders - a very
rare occurence on a Stag!  The pedal is rock solid.  I also have silicone
fluid in all the other cars in my family including my Vitesse Sport and
gives good results in all cars with excellent 'feel' and no sponginess.

As far as I can see there are two drawbacks.  The initial cost of silicon
fluid is high (but is saved in the long run as it doesn't have to be
changed with monotonous regularlity) plus it is considered desirable to
change all the brake seals when first using silicone fluid - but is NOT
essential.  This latter problem will not affect you of course.

I also thought I would check out what racing teams use.  Saloon racers on a
budget use conventional DOT 5 ester-based fluid - but change it every
race-day!  Sports cars, professional saloon racers, F1 and CART use a
hydraulic fluid under high pressure to cope with the excessive heat - while
Citroen used engine oil in their more technically advanced road cars up
until recently!!

Mike Wattam
Triumph Stag Register