Brake Fluid Information


BRAKE FLUID - What is it and why is it so important?

Brake fluid is the means by which foot pressure on the brake pedal is transferred to the brake pads and discs to slow or stop a vehicle. The brake fluid lives in a pressurized world within the master cylinder, brake hoses and brake caliper. The pressure placed on the brake pedal is transferred by the master cylinder compressing or forcing the brake fluid along the brake lines to force the brake caliper to close the disc pads onto the disc rotor. Brake fluid is an easily neglected critical safety area of any motor vehicle. We check tyre pressure, oil levels and other fluids at regular intervals and so we should do the same with the brake fluid. Because our brake fluid operates in this pressurized world it is imperative that the fluid level should not alter. If it does, our pressurized brake system becomes unsealed and therefore its performance is reduced. Brake hoses being made of rubber; deteriorate over time, as can the rubber seals and fittings, leading to a softer brake pedal, more aggressive foot force to stop or even brake failure. 

Not only should we monitor the level of brake fluid, we should regularly replace it. 

  • How often? For optimum performance – annually. 
  • Why? Brake fluid breaks down over time and absorbs water even though the system is sealed. Fresh brake fluid when new has maximum compression characteristics, but over time and uses it loses compression though changes in its composition and make up. 

How is this possible in a sealed environment you ask?

One of brake fluids most important characteristics is in fact its ability to absorb water! It is designed to absorb water! Diffusion allows moisture in the air to permeate microscopic pores in the rubber brake hoses and the various seals in the hydraulic brake system. This moisture would then rot out the internals of our brake system if it wasn’t absorbed by the brake fluid. In extremely cold weather it also stops this water/moisture from freezing in the brake system. This feature comes at a cost, which is, that water contaminated brake fluid reduces its performance. But brake fluid composition and therefore its effectiveness can also be altered by its working environment, because the brake system generates extreme temperatures, some of this is transferred off the disc pad and rotor into the brake caliper holding the disc pad and this heats up the brake fluid that flows within the brake caliper. To give you some idea of this in action, should your brake system have 3.7% of water trapped within its brake fluid, the boiling point level of your standard brake fluid is reduced from 205 degrees Celsius to 140 degrees Celsius a thirty percent reduction! So we now know that moisture will reduce the effective boiling point by almost 1/3. There are many different types of brake fluids with many differing effective temperature ratings to handle this event. 

So apart from changing my brake fluid regularly should we look to upgrade the fluid to one of these with a higher/greater temperature tolerance? 

The rating of the fluid your car was delivered with should be maintained unless you upgrade the internal components to handle this upgrade. All seals, brake hoses and fittings are designed and tested relevant to the chemical composition of the brake fluid to be used. Simply replacing with a higher grade brake fluid (replacing Dot3 with dot 4 for instance) gives rise to the impact the slightly different composition (the borate ester) might have on your braking system. The viscosity difference (thickness) can effect the wear rates on seals etc and cause squeaks to develop. So it’s best to stay with the same brake fluid type, but maybe go for a higher operating range within the same dot fluid, rather than jump up to a higher dot rated fluid. 

What do the various ratings of brake fluid mean?

There are three main classifications of brake fluids which are known as “Dot 3, Dot 4 and Dot 5” The Dot stands for Department Of Transport and is an American based standard and runs to 14 different requirements, both physical composition and operating, under which each must be made. The most important is the measurement of its boiling point characteristics and these are measured within two ranges, being its dry boiling point and its wet boiling point. Dry boiling point is the point at which the fluid boils when first used out of the bottle and wet boiling Point is a measurement based on 3.7% water absorption in the brake fluid and at what point it starts to boil. As we know from above, water is absorbed into the brake fluid by design and it traps a lot of heat from the brake operation. These two events alter the effectiveness of the brake fluid and the minimum operating temperatures as described by the Department Of Transport regulations ensure the fluid is still capable of functioning safely. 

  • DOT 3 - Usually glycol ether based with a minimum dry boiling point of 205 degrees Celsius and a minimum wet boiling point of 140 degrees Celsius (with 3.7% water content as discussed above). 
  • DOT 4 - Also glycol ether based with a touch of borate esters to increase it’s immunity to water absorption. Dot4 must have a minimum of 230 degrees Celsius dry and 197 degrees Celsius wet. 
  • DOT 5 - Silicone based and must have a minimum boiling point of 265 degrees Celsius dry and 180 degrees wet. Being silicone based this type of fluid flows more easily through the pressurized braking system Giving greater braking performance and thereby reducing heat build up. The disadvantage is that by it’s nature being more compressible it allows more room for air to be present within the fluid (air becomes trapped within its molecular structure).

There is a fourth classification of brake fluid being DOT 5.1. Recent innovations has lead to the development of a Glycol ether based fluid that now meets the characteristics as required under the industry standard DOT 5. It has the same dry and wet minimum boiling points and is basically DOT 4 fluids with higher boiling points. Also being Glycol ether based it doesn’t share the negative feature of silicone based fluids or dot 5, as air is not trapped within the silicone. These are also known within the industry, sometimes, as dot 4 plus.

  • DOT 5.1 is therefore the best of the best but it comes at a cost differential to DOT 3, 4 & 5 fluids. 

Why should I buy brake fluid in small containers and not in bulk like engine oil?

Buying brake fluid in small containers and not using leftovers is paramount to having a safe and Effective braking system as once opened, the contents are drawing moisture and air and losing its compression and therefore its optimum performance characteristics, the same as if sealed within your hydraulic braking system as described above.

Bleeding your brake system.

This is the method by which fluid is replaced within the pressurized brake system and air is eliminated. Replacing all the brake fluid throughout the lines, cylinders and calipers must be undertaken with extreme care as any trapped air will decrease significantly the operating performance. The objective here is to obtain an air-free brake system. 

Performance and racing / summary. 

There is not a brake fluid available that will allow you to run indefinitely without periodic changing or bleeding. When racing or doing laps the brake fluid should be replaced both before and after the days racing (each event would be better) and using a cool down lap before stopping, will assist greatly in preventing boiling, as it will avoid the heat soak when the airflow stops. The brake fluids job is to provide you with a consistent stable performance and regular bleeding and replacement, will ensure optimum operating performance and eliminate the possibility of brake failure, when you need your brakes the most- that all important emergency or hard braking situation.