Courtesy of Steve, a fellow Hayabusa owner.

Brake Fluid

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 rating.

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 that straightforward.
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 type.

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 = non-Silicon!


Do use Decent DOT 4 in your Motorcycle braking system. (Except modern Harleys)
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 it.
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.