Brakes – Significance of Brake Fluids

Brakes – Significance of Brake Fluids

The acceleration of a vehicle from stop to top (speed) is an exhilaration in itself, but once at high speeds we need brakes to compliment and control the vehicle. The vehicles will keep getting faster and faster and along with them their brakes will have to match equally. When it comes to a performance vehicle, brake maintenance becomes a crucial factor. From brake pads, brake lines, cleaning and to using the right brake fluid, all matters a lot. But what we are going to talk about today is the Significance of Brake Fluids. We usually check the tire pressure, amount of fuel and working of important parts before heading out, but what often gets overlooked is the brake fluid. Let us begin then, shall we?

what is brake fluid.jpg

The function of Automotive Brake Fluid is to transmit the pressure applied by the brake pedal to the wheel brake cylinders, that pressure originates from the master cylinder. The brake fluid basically provides an in-compressible medium and it efficiently does the job till it is fresh and in the right working environment. Problems arise when working temperatures rise or the age of Brake Fluids increases. When used excessively, brakes can heat up the brake fluids to their boiling points resulting in failure.

Dry and Wet boiling points:

  1. Dry boiling point: The boiling point of a brake fluid that has been freshly opened from its container to use has a dry boiling point (0% water).
  2. Wet boiling point: With time and use, brake fluids absorb water from the atmosphere. So for an old brake fluid, wet boiling point is applicable. This is measuring taking the water percentage to be 3.7%.

Say, a bike that has been using a brake fluid for a couple of years will have a wet boiling point. Whereas if in the same bike, the old brake fluid is flushed out and filled with a new one, then dry boiling point becomes applicable.


It can be said as the thickness of a fluid. Or the resistance of a fluid to flow. For example, water flows easily when compared to honey. It is also seen as a thinner fluid as opposed to honey. Hence honey is more viscous or has a higher viscosity when compared to water.

Types of brake fluids:

In general, they are categorized into polyglycol (Water absorbing- Hydrophobic nature) and silicone (Water repelling- Hydrophobic nature) based brake fluids. DOT 3, DOT4, DOT4 (LV) and DOT 5.1 are polyglycol based fluids whereas DOT 5 is silicone based. Lets understand them better:

1. DOT 3 Brake fluids:

As already stated, they are polyglycol ether based. Their preference is because of their affordability. They have a dry boiling point of 140 Deg Celsius and a wet boiling point of 205 Deg Celsius. They are generally used for regular driving vehicles and light weight trucks. They are capable of absorbing 1% to 2% of water from the atmosphere in a year’s use.

2. DOT 4 Brake fluids:

These have a higher dry and wet boiling points compared to DOT3. With a dry boiling point of 155 Deg Celsius and wet boiling point of 230 Deg Celsius. They also have a higher viscosity. (They might have higher boiling points but they absorb more water and need replacing more often than DOT3).

3. DOT 4(LV) Brake fluids:

Since DOT4 fluids have higher viscosity than DOT3, DOT4(LV)s were introduced. Where LV stands for “Low Viscosity”. Anti-Lock Brake Systems (ABS) have micro valves that pulsate and push the brake fluid multiple times per second. Higher viscosity makes it harder for the valves to function since they require more effort. For this reason, DOT4 (LV) does the perfect job by reducing load of the micro valves. Thus making it a preference for vehicles with ABS.

4. DOT 5 Brake fluids:

These fluids are silicone based and hence have negligible water mixing. DOT 5 fluids are specifically designed for vehicles that are not operated for long periods of time. Years even. The perfect examples are military vehicles and vintage/antique vehicles. They aren’t suitable for modern day cars because of how much they can be compressed.

5. DOT 5.1 Brake fluids: 

These are again polyglycol based and offer the highest dry and wet boiling points. They are used in high performance vehicles making them the best on the market.

  DOT 3 DOT 4 DOT 4(LV) DOT5 DOT 5.1
WET BOILING PT. (C) 140 155 170 180 180
DRY BOILING PT. (C) 205 230 265 260 260

-40 C

1500 1800 700 900 900

*The table shows the minimum dry and wet boiling points. Different manufacturers can have higher boiling points in any of the given brake fluid


Key factors when looking at brake fluids: 

a. The fluid needs to be non-compressible to transfer pedal force to the brakes themselves.

b. Needs to have a low-viscosity to be compatible with ABS.

c. Needs to have good lubricity.

d. Needs to have corrosion-resistant characteristics.

e. Needs to have high boiling points.

Keep an eye out for these points and you’re all set.



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