Brake fluid sucks! Literally.

Brake fluid is hygroscopic meaning "Readily absorbing moisture, as from the atmosphere."

This is a deliberate design parameter for brake fluid as it means that any moisture or condensation in the brake system won't collect and either cause rust in the pipes or pool and freeze in cold climates which could result the brakes not working or split pipes due to the expansion of the ice.  Instead the moisture is absorbed and effectively diluted to avoid these problems.

The moisture gets in mainly through the breather cap on the brake fluid reservoir, which has to breathe as the brake fluid levels go up and down when you press the brake pedal.  Some moisture also passes through the rubber flexible hoses leading to the wheels.

The problem with this, is that brakes operate at high temperature. When you stop the car, you are converting the motion of the weight of the car mainly into heat via friction between the brake pads and the rotors (rotating disk) attached to the wheel.

While most of the heat is dissipated by the vented rotors, a lot of this heat is still transferred to the brake calliper which holds the pistons that squeeze the pads against the rotor, in turn heating the brake fluid.

Brake fluid therefore has a high boiling point, typically 2-300 degrees Celsius, but water has a boiling point of only 100 degrees Celsius. Small amounts of water absorbed into the brake fluid reduces the effective boiling point down to typically 140-200 degrees Celsius.

Now you can start to see the problem.  If the fluid boils, you can get pockets of steam in the brake system which is the same as having air in the brake lines. These bubbles are compressible unlike the brake fluid, so when you press the brake pedal, you end up using energy to compress the bubbles instead of using that energy to press the pads against the rotor. The end result is that the pedal now feels spongy and the brakes don't have enough pressure to stop the car.  This phenomenon is commonly known as Brake Fade. and is very dangerous.

For this reason alone, it is important to change or flush your brake system at a maximum of 2 years, more often if you do track days.

Because of the absorbed moisture problem, don't buy too much brake fluid as most containers allow some moisture in when being stored. I buy 4 litre packs, but I use them in less than a year as I do track days and change the fluid often.

What is the DOT?
DOT stands for the US Department Of Transportation who issue the specifications for brake fluid. Brake fluids have a "Dot x" design specification, i.e. currently  DOT3, DOT4, DOT5 and DOT5.1.  There are a lot of parameters that make up the specification, but the main one is usually the boiling point of the fluid both when it is "dry" - new with no moisture, and when it is "wet" with absorbed moisture.  Studies have generally show that a car will absorb approximately 2% moisture per year and 3% moisture lowers the boiling point to close to the minimum allowed level.

Besides not boiling, brake fluid needs to have some other specific qualities:

• Mustn't freeze or thicken at cold temperatures.
• Must not compress.
• Must flow freely through rather small passages.
• Must not corrode or react with materials in the brake system.
• Provide lubrication to the moving parts of the brake system.
• Its properties must remain stable for extended periods.
• Compatible with other glycol fluid chemistries.
• Cannot decompose or form gum or sludge in the system.

With the exception of DOT5, brake fluids are made from a Glycol based mix and therefore have similar characteristics though with higher specifications e.g. boiling point.   DOT5 however is a Silicon based fluid which is incompatible with the other fluid types and should not be used as it has it's own set of problems.

A good quality DOT4 is adequate for daily and spirited driving and there are newer Super DOT4 fluids that have the DOT4 specification but with higher boiling points that are excellent for track days as well.

Comparison Boiling Point Chart showing some popular brake fluids.

Dry Boiling Point Order
Dry Deg C	Wet Deg C	Product
321	216	Neo-Synthetic Super DOT 610
312	216	Motul RBF600
310	270	Castrol SRF
310	210	AP Racing 600
286	184	Castrol Response, Super Dot 4
280	200	ATE Superblue/TYP200
275	150	AP Racing 551
265	185	Motul 5.1
261	173	Valvoline SynPower
230	155	DOT4
205	140	DOT3
Wet Boiling Point Order
Wet Deg C	Dry Deg C	Product
270	310	Castrol SRF
216	321	Neo-Synthetic Super DOT 610
216	312	Motul RBF600
210	310	AP Racing 600
200	280	ATE Superblue/TYP200
185	265	Motul 5.1
184	286	Castrol Response, Super Dot 4
173	261	Valvoline SynPower
155	230	DOT4
150	275	AP Racing 551
140	205	DOT3

I use the Castrol Response, Super Dot 4 which has never let me down on track days and is probably the best value in Australia at around $35-40 for 4 litres.  I change my brake fluid a couple of times a year as my local track at Mallala is notorious as the hardest track on brakes in Australia.