Can you use any fluid in the braking system?

No you cannot. The fluid in the braking system must be a designated brake fluid which meets with certain specifications - DOT3, DOT4, DOT5 or DOT 5.1 are the most common and your owners manual will inform you of the fluid used as standard in the system. Don't use aircraft hydraulic fluid (even if you can get hold of it) - it isn't the brake fluid equivalent of Avgas and do not use the mineral based fluid formulated for Citroens either.

What types of brake fluid are available?

There are basically two types of fluid: glycol-based and silicone-based. You can use one or the other but NEVER mix the two fluids. If you want to use a different fluid than the one in your system (glycol to silicone / silicone to glycol) then you will need to flush the system before changing. Brake fluid is available in different specifications to meet American Department Of Transport (DOT) requirements. These DOT regulations are occasionally updated but basically the higher the DOT rating the higher quality, higher boiling point and more expensive the fluid. DOT3 is the basic brake fluid. DOT 4 is a higher standard and both of these fluids absorb water - they are hydroscopic but DOT4 fluid contains additives which prevent water affecting the fluids performance to the same degree. DOT 5 was originally formulated for silicone based brake fluid although there is now a new DOT5.1 standard for glycol based fluid which has some of the desirable properties of silicone fluids. Both types - glycol based and silicone have different pros and cons though.

What are the boiling points of brake fluids?

Brake fluid works in a harsh environment being close to the heat generated by the braking system the fluid heats up and under extreme conditions such as racing this can lead to the fluid boiling. This manufacturer found that by increasing the boiling point the fluid performed better in use as when fluid boils it leads to cavitation within the system and loss of braking pressure. The boiling points are as follows:

Fluid Type Minimum Specification Good Brand Minimum Specification
Dry Wet Dry Wet
DOT 3 205°C 140°C 220°C 150°C
DOT 4 230°C 155°C 260°C 170°C
DOT 5 260°C 180°C 270°C 190°C
DOT 5.1 260°C 180°C 270°C 190°C

Wet figures above show the drop in performance when the fluid has a certain water content (less than 3%). For example when DOT 5.1 is contaminated with 3% water (which is considered a lot) performs almost as well as basic DOT 3 in prime condition. The most important thing to do is regularly change your fluid - we suggest every three months but manufacturers suggest at least once every twelve months. After six months use a typical DOT 3 fluid may have had it's boiling point lowered from 205°C to about 165°C

What are the pros and cons of fluids?

DOT 3 and DOT 4 are hydroscopic - they allow water to be absorbed into the fluid. This means that when heated up a fluid with water in it will boil more quickly (water boils at a lower temperature than brake fluid) which will lead to cavitation. DOT 5 does not absorb water and will not strip paint work but as the water is not absorbed it simply sinks to the bottom of the system which is usually the caliper where operating temperatures are very high. Water in the caliper is bad as it can cause corrosion within the caliper although silicone fluids contain corrosion inhibitors and as water has a lower boiling point than fluid (100 Degree's C) there is more chance of boiling under relatively light braking. Glycol based fluids can strip paintwork where as silicone based do not. Silicone fluids also have a very long life which is why it is used in military vehicles which may be stored for years without use but be expected to be ready for action at a moments notice.

Do -02 lines give more pressure than -03 lines?

No - the fitting of the smaller dash 2 hoses sold by some manufacturers makes NO difference to the pressure produced at the caliper as is commonly believed. There are no substantial benefits associated with the use of dash 2 hoses except that the manufacturer can charge you more for them. The only way to increase pressure coming out of the system is to increase the pressure going into the system. 500 psi in equals 500 psi out - Fluid cannot be compressed to a lesser volume, no matter how high the pressure and Pressure is equal over all surfaces of the containing system.

What is the difference in performance between two full length front lines and an over the mudguard line kit?

Nothing except for the way the system looks. Both systems operate in exactly the same way and give you the same performance. As the system is a closed hydraulic system without air in it when the fluid moves at one end the same movement takes place at the other end. Because they have different lengths doesn't mean that the system will work slower at one end than the other. Imagine pushing two sticks away from you which are both 6" long - the ends both move at the same time. Now imagine pushing two sticks - one is 6" and the other is 36" - the ends still both move at exactly the same time. So the over the mudguard gives the same performance as the 2 full length - it's just which you prefer.

What's better - three front lines using the oem splitter, 2 full length fronts or 2 fronts with one over the mudguard?

As we have said above the three systems give the same performance but the three line front has an additional 2 possible leak points in the system, is heavier and harder to bleed than 2 front lines. It is easier to fit though as you follow the oem setup. Another thing to remember is do you race or use the bike on track days ? If so go for the 2 full length front as in the UK one of the ACU rules state that you are not allowed an over the mudguard system unless it is an original system.

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