Courtesy of Steve, a fellow Hayabusa owner.
Brake fluid comes in two major types....
Ester Glycol and silicon based. Ester Glycol can be mineral, semi or fully
synthetic all of which can be safely mixed within the bounds of their DOT
Racers often use Silicon fluid as it does not compress very much and gives
you a firmer/more consistent lever.
Unlike Ester Glycol fluid, Silicon fluid is not hydroscopic - that is it does
not absorb moisture. The perfect fluid than huh? No.
Moisture will enter your brake system whether you like it or not and unlike with
Ester based fluids it will not be dispersed but will concentrate at the lowest
point in the system.
This concentrated moisture is highly acidic and will make short work of the
pistons and seals. Normal brake fluid gradually discolours and only becomes
slightly acidic over time - the spongier brake lever thanks to the moisture in
the system gives you ample warning that a fluid change is needed whilst
gradually lowering the boiling point of the fluid.
Joining the Dots...
DOT standards for brake fluid stipulate a minimum wet and dry boiling points
for the fluid, dry is brand-new fluid out of the tin and wet is fluid
artificially contaminated with a defined percentage of water, but things aren't
Silicon fluid does not absorb water - therefore the 'wet' test results look
incredibly encouraging showing little if any degradation - that's because indeed
there is none - the fluid is still pure. However, you have a growing pool of
water sitting in your braking system under the less dense Silicon fluid, this
water will of course boil at just above 100 degrees (higher temp is thanks to
the fluid pressure when the brakes are applied) or at 100 degrees exactly when
you let the brakes off. The water will rapidly turn to steam. In sufficient
quantities this will cause either total brake failure or a locked on brake
thanks to the expansion of the steam.
If you really must use Silicon fluid (why?) - bleed your system every other
week! Plus every seal in the brake system should be replaced if changing fluid
DOT 2 (not seen for years),3,4 and 5.1 fluids are all compatible and mixable
with each other, however you should never put a lower DOT fluid in the system
than the manufacturer specifies.
What's the issue with DOT 5? Can I use it?
No, not unless you ride a Harley! DOT 5 originally meant Silicon fluid but a
Ester Glycol fluid matching this specifications (viscosity and minimum heat
rating soon appeared.) then the confusing high performance Ester fluid standard
of 5.1 was formulated, people naturally started mixing the two - ouch. Never the
twain should meet.
The Americans dropped all mention of DOT 5.1 and called 5.1 mineral DOT 5 and
real DOT 5 Silicon DOT which must also carry purple die to differentiate it.
That means in the states you can buy mineral fluid marked as DOT 5 which is DOT
5 or DOT 5.1 - again the two must not be mixed!
DOT 5 was formulated to work with car ABS systems and that's where it should
stay, The viscous nature of DOT 5 fluid means that air bubbles are hard to
remove from a bike brake system and the fluid itself is not friendly to seals
designed for DOT 2,3,4 or 5.1.
In the UK we very rarely use the confusing DOT 5.1 moniker either, rather
than risking using the wrong fluid stick with DOT 4. if you want better fluid
performance buy high performance DOT 4 such as Castrol Response, this is a DOT
5.1 Synthetic based Ester Fluid. (available from Sainsbury's Homebase.)
Brits do not die Silicon fluid purple, the only way to tell is to pour some on
some paint-work and leave over night - no paint left in the morning =
Do use Decent DOT 4 in your Motorcycle braking system. (Except modern
Do bleed system regularly especially before and after the bike is to be stored
this will extend the life of the pistons in the callipers.
Do use a higher compatible DOT standard than the OE specification if you wish.
Do wear barrier cream or gloves when dealing with non-silicon fluid.
Do not Use silicon fluid of any type on the road - the risks are not worth
Do not confuse DOT 5 and 5.1, buy performance DOT 4 to be safe rather than sorry
- this stuff IS DOT 5.1.
Do not use fluid from a bottle that has been opened, as an experiment get half a
bottle of fluid and mark the level - take the top off and leave in the open air
for an hour - come back and recheck the level it will have gone up from the
absorption of incidental moisture in the air! Overnight it would probably over
spill! (doesn't apply to silicon fluid.) The boiling point of fluid that has
been opened is dramatically compromised.
Do not leave the top off the bottle/master cylinder any longer than is strictly
necessary, see above comment on moisture!
Do not get DOT 2,3,4,5 (mineral) or 5.1 on your skin, It is high in hydrocarbons
small enough to pass straight through your skin into the blood stream. Your
Liver will not be too pleased. Hydrocarbons are Cancer forming.
Do not let your pads wear too much, the lack of friction material will increase
brake temperatures and put more strain on the fluid.
Do not Use Alloy banjo bolts - leave em to the racers. These can fatigue fail in
road applications. Stainless Steel is still best if less pretty!
Do not allow non-silicon fluid near hot surfaces or a naked flame. Amazingly the
flash point of Ester Glycol brake fluid is lower than that of Petrol! And when
it burns, it burns very hot letting off rather nasty fumes in the process.
Do not ignore this article and listen to so called "experts" I've
heard of race shops telling people to use DOT 5 to improve their brakes! Stupid
at best - fatal at worst.