Brake Fluid - DOT-3, DOT-4 and DOT-5

by Andrew Automoto21. November 2013 10:29

We all know that if you push down on your brake pedal your vehicle slows and eventually stops, but how does this happen? What happens under your bonnet to stop something as big and heavy as your car? Well a big part of working your braking system is Brake fluid:

Brake fluid, is probably the most neglected of all your vehicles fluids.  Brake fluid is the hydraulic fluid that runs through the cylinders from your brake pedal to your pistons which then push the pistons against the brake pad, causing the brake pads to rub on the rotor and cause friction there by slowing your car down, and eventually stopping it.

Brake fluid has a very high boiling point, however with prolonged breaking i.e. going down a steep hill, the fluid can come close to boiling point.

Now for a little science lesson: When fluid boils it becomes a gas, this means there is no force pushing the pistons out to push the brake pads therefore stopping your vehicle. Brake Fluid, comes in three different types affectionately known as DOT-3, DOT-4 and DOT-5.

DOT-3 and DOT-4 are glycol-based brake fluids, and the most common types of brake fluid used. This means they absorb water from the air, which decreases there boiling point, or in the colder months turning the water into ice crystals which can lower your brakes stopping abilities. This is just one of the reasons why it is important to keep your break fluids in an air tight container, and to make sure you don’t open your brake fluid reservoir too often.  DOT-3 and DOT-4 are highly corrosive brake fluids, and will eat paint, so make sure you don’t get any on your car.

DOT-5 is synthetic, which means it does not absorb water, this means the boiling point remains relatively stable, but it also means that any water that gets into your brake system will form pure water pockets, which could cause corrosion to your brakes. DOT-5 is mainly used in race cars, and is rarely used in vehicles with everyday use.

All 3 types of brake fluid are different and the different types of brake fluid should not be mixed. Mixing brake fluid, even though DOT-3 and DOT-4 are the same base, can be dangerous, as they can react badly to each other and corrode your brake system.


Brake fluid, should be changed and flushed, every two years or so. While brake fluid doesn't go bad, it will eventually start to absorb water, which is inevitable in the long run, as the system is vented, this is why it is important not to open the reservoir often, the more water you can stop from absorbing into the brake fluid the better.

Flushing and changing your brake fluid, is not an easy job, and if not done properly can cause serious damage to your brakes, and compromises your, and other road users safety. That is why if you are reading this, it is Highly recommended that you contact your local authorised service centre or dealership, to discuss having your brakes checked, and your brake pads and fluid changed.

You can find your closest authorised brake specialist at www.carservice.com.au/brakes

If you do decide to change your brake fluid yourself, it is important that you buy the same type of Brake fluid as has previously been put in your car, and it is important that you use well known brands.

Some of the best known brands in Australia are:

  • Shell
  • Polex
  • Lucas - now known as TRW
  • Knorr-Bremse
  • Castrol
  • Nulon
  • Penrite

 

For information on your vehicles bake pads see - Brake Pads what they do and when to change them

If you do decide to change your own brake fluid see - How to flush brake fluid

 

 

‘The information provided in this article is for general advise only.  It is not intended for instructional use.  Use of the information is this at your own risk.  We recommend you seek the advice of a trained professional.  ’