Brake Fluid – What should I use?
Brake fluid comes in many specifications, shapes, sizes, types and costs – but understating brake fluid is actually quite straight forward.
We just need to focus on a few questions:
- What are the different specifications available? Incl. Dot ratings and boiling points (wet and dry)
- Which of these specification suits my needs?
- How do I make sure my brake fluid is in good condition?
Once we know these few things, it is time to select the right fluid!
Also, it is important to note there is a big difference between brake fluid requirements in standard road cars, and competition vehicles. The needs are very different, and the product is different too. Road vehicles don’t tend to introduce very much temperature into the brakes system compared to competition vehicles, so only need base specification fluids that are ‘designed to a cost’. We will push those to the side and focus on competition fluids only.
WHAT ARE THE VARIOUS SPECIFICATIONS AVAILABLE?
Let’s start with DOT ratings which are standards set by national standards, primarily in the US and are adopted the world over now.
You will hear about DOT3, DOT4, DOT5 and DOT5.1 but we will basically remove DOT5 from this discussion because it is silicone based and very rarely used now-days in any sort of performance or competition situation. Mainly it is used in some military applications and maybe some Harley Davidsons!
DOT5 Silicone based fluid is older technology and should never be mixed with any synthetic fluids. It was originally designed due to silicones ability to not allow moisture. However, with new technology, DOT5.1 was developed to offer similar performance and to meet the same requirements without the need to use silicone.
This leaves us with DOT3, DOT4, and DOT5.1 which are glycol-esther (synthetic) fluids. They all have their own minimum boiling point and compressibility levels. For example, the minimum wet boiling point (explained later) requirements for DOT standard fluids are:
DOT3 = 140°C
DOT4 = 155°C
DOT5.1 = 180°C
The original grade DOT 3 was the standard brake fluid for many years, and it still is in some countries, particularly North America. However, DOT 3 grades tend to suffer from steep boiling point decline over time, and DOT 4 fluids were developed to counteract this. They contain polyglycol ether esters, which chemically bind the water and dramatically slow the decline in boiling point.
Many fluids used in club level motorsport will tend to be DOT3 because they are cheaper and, in many cases, meet the requirements of these competitors.
DOT 5.1 fluids are a development of DOT 4 and can offer higher boiling points (and retaining them even better than DOT 4 in service) but also having a much-improved viscosity (thickness) at low temperatures.
Interestingly, some of the very best racing brake fluids including Castrol SRF are DOT4 for overall performance reasons.
WHAT SPECIFICATION SUITS MY NEEDS?
In summary, if you are only using your car on the street, you should look to DOT3 or DOT4. If you use your car in competition, use DOT4 or DOT5.1. For more advice, contact us.
HOW TO MAINTAIN YOUR BRAKE FLUID
Selecting the right brake fluid for your needs is very important, but arguably more important is maintaining the fluid.
A common misconception by car owners is that the brake fluid in your brake system somehow ‘circulates’. It doesn’t. Generally, the fluid in the reservoir stays there, and the fluid in the caliper also stays there. A quick top-up of the fluid in the reservoir will do nothing to keep it fresh where it matters most – in the brake caliper.
The pressure seals that surround the caliper piston are covered in a fine layer of brake fluid, and it is this fluid in this area that exerts the pressure on the piston, which in turn pushes the pads against the disc. Therefor this is the fluid that cops all the punishment and heat. This is the fluid that needs to be fresh.
Bleeding your brakes before you use your car in a competition environment to make sure the fluid inside the calipers is fresh. This effectively gives you brake fluid that has its highest possible boiling point. This gives you a better chance of avoiding the fluid fade mentioned earlier.
Dry boiling point and Wet boiling point. What are they?
Brake fluid is hydroscopic which means it will naturally want to absorb moisture from air. Despite the brake system of (most!) cars being completely sealed, moisture content does happen. As this happens, the brake fluid’s ability to reach the high boiling points becomes less. So brand new fluid with 0% moisture content is the best performing brake fluid. This is when dry boiling point can be measured. When the fluid is older and has a small percentage of moisture (even 1% will reduce performance), the wet boiling point becomes relevant.
As an example, the image above is of the brand new CIRCO MF1200+ Racing Brake Fluid. This product is DOT4 and is designed for competition and high-performance street use. It has a dry boiling point of 328°C and wet boiling point of 204°C.
For cars that don’t get regularly serviced such as everyday road cars, the brake fluid is only checked now and then, and, is less of an issue than in competition cars due the lower levels of stress they are put under. But for competition vehicles, the higher the boiling point is, the more you can lean on the brake system before you get any sort of ‘fluid fade’, which we will talk about shortly.
Difference between Pad fade and Fluid fade
Many people mix these two up! It’s not uncommon to hear someone at the track complaining about the wrong thing when they have brake issues.
“My pads are no good because my pedal is going long!”
“My car won’t stop no matter how hard I press!”
Unless there has been some sort of mechanical failure in your brake system, the only way a brake pedal can ‘go long’ – which is a horrible feeling – and can end in disaster – is by pushing your brake fluid past its boiling point. If the fluid is over-heated it will develop small gas bubbles which unlike the fluid is compressible. This compression is what will cause the long pedal known as brake fluid fade.
If your pedal is still solid or ‘normal’ but the car won’t stop, this is brake pad fade caused by the temperature levels of the friction material too high. This also can cause gas to form but this time between the disc and pad which reduces friction, hence stopping power.
Can brake fluids be mixed?
Yes, brake fluids conforming to DOT 3, Dot 4 and DOT 5.1 can safely be mixed although the performance of the higher specification products will be diluted. Silicone (DOT 5) and mineral-based brake fluids should never be mixed with any other types.
I have an old bottle of brake fluid. Is it still O.K. to use?
If the bottle has been previously opened it is probably better to discard it and buy new, unless you know the product is less than three months old and the bottle was resealed tightly.
For unopened bottles do you know how old it is and how it has been stored? If it is still in a sealed bottle, less than 18 months old and stored in a cool, dry place, then it is likely to be quite safe to use.
