Viscosity

Brake fluid viscosity is a measure of how thick the fluid is, especially at low temperatures. It's important for your brake fluid to maintain a constant viscosity across a range of temperatures, especially very cold temperatures. Brake fluid is developed to be thin so it easily flows through the ABS system and your braking systems. Brake fluid protects the metals used inside the braking system such as calipers, wheel cylinders, master cylinders, and ABS control valves. The viscosity of your brake fluid will be generated from the wet and dry boiling points of your chosen brake fluid, along with the amount of time it takes for the fluid to reach a predetermined amount of space. Below you can find the minimum requirements by law and under legislation, for the DOT regulations which determine how your viscosity will be determined and regulated.

The minimum specifications for DOT 3 brake fluid include:

Wet boiling point: A minimum of 140°C (284°F)

Kinematic viscosity at -40°C: A maximum of 1500 mm²/s

Kinematic viscosity at 100°C: A minimum of 1.5 mm²/s

The minimum specifications for DOT 4 brake fluid include:

Wet boiling point: A minimum of 155°C (311°F)

Kinematic viscosity at -40°C: A maximum of 1800 mm²/s

Kinematic viscosity at 100°C: A minimum of 1.5 mm²/s

The minimum specifications for DOT 4+ brake fluid include:

Wet boiling point: A minimum of 155°C (311°F)

Kinematic viscosity at -40°C: A maximum of 750 mm²/s

Kinematic viscosity at 100°C: A minimum of 1.5 mm²/s

The minimum specifications for DOT 5 brake fluid include:

Wet boiling point: A minimum of 180°C (356°F)

Kinematic viscosity at -40°C: A maximum of 900 mm²/s

Kinematic viscosity at 100°C: A minimum of 1.5 mm²/s

The minimum specifications for DOT 5.1 brake fluid include:

Wet boiling point: A minimum of 180°C (356°F)

Kinematic viscosity at -40°C: A maximum of 900 mm²/s

Kinematic viscosity at 100°C: A minimum of 1.5 mm²/s

So why do people run DOT4 still, on track cars? This is mainly due to the fact that the minimum requirements are minimum requirements for a reason. The factory cars are shipped and delivered with DOT4 as standard. As from the above information, you can see that the wet boiling point generally doesn't move throughout the regulations. This means that you do not really need to move to any other DOT variation for your brake fluid. You would be better off determining the correct wet and dry boiling points for your use case, if you are using the braking system much more. Of which, would be a bigger determining factor within your decision making.

What Brand Of Brake Fluid Should I Use?

Generally as per regulations, the brand of brake fluid you use shouldn't be all that different from brand to brand as they have to meet legislated requirements. The best way to determine what is best for you would be to look at the wet and dry boiling points.

How Much Fluid Do I Need?

The fluid levels of your braking system will be shown on your manufacturer handbook. In most cases for the MQB/VAQ platform, the braking system is mostly 1.5L. Majority of brake fluids will come in 500ml bottles, meaning you would need 3 bottles for a full flush and replacement of the system. We would advise to use and buy 4 bottles if you have a fully drained system as you may need to bleed air out past the point of the full level. Although you can reuse drained fluid from the calipers, we do not recommend this.

How Do I Bleed My Brakes?

The correct way to bleed any VAQ vehicle stated in the OEM manual is as follows;

1- Front left brake caliper

2 - Right front brake caliper

3 - Left rear brake caliper

4 - Right rear brake caliper

Please bare in mind that the above order of bleeding is in the VW manual and should be adhered too. However a mechanic from a professional trade may bleed in a different way due to generational knowledge. This sequence would be as follows (Provided for UK cars with the ABS module on the passenger side);

1- Right rear brake caliper

2 - Left rear brake caliper

3 - Right front brake caliper

4 - Front left brake caliper

If you still suffer from any brake fade on track after performing a full bleed on your system with high temp brake fluid - read more here.

Wet Boiling Point

The wet boiling point stated by the manufacturer for your chosen brake fluid will be determined under their rigours testing before heading to market. This boiling point generally gives you an indication of how much heat the fluid can take once it has started to degrade. Degradation is usually at it's highest point once around 3.7% water has entered the system. Although this does not seem high - any water or air within your braking system will cause issues relating to braking force, pedal feel, stopping distances and ABS modulation. Water within the brake fluid can cause corrosion, expansion and gas pockets within your enclosed system causing the above issues and potential failures.

Dry Boiling Point

The wet boiling point stated by the manufacturer for your chosen brake fluid will be determined from factory, as per what is poured into the bottle you have purchased. This means that the temperature that the fluid should ultimately reach from new is as per what is stated under the dry boiling point. This is the temperature limit provided within an opened bottle before it is inserted into your braking system. This temperature limit should remain consistent within your braking system until it meets the point of degradation. The fluid will start to lose the dry boiling point ceiling, once it hits its temperature limit and/or weather conditions push the fluid out of its temperature scope limit. 

pH Value

Degradation of a brake fluid can be determined by the amount of pH Levels within the brake fluid. pH Levels are determined upon how acidic or alkali the brake fluid is from the way the manufacturer has made up its compounds within your chosen brake fluid. Generally a pH value of 7.0 is a very strong acid level and can accelerate corrosion and degradation of the fluid as well as other parts of the braking system. As long as you change the fluid as per recommended then you may never see any issues. The only issues you may experience is once your fluid has degraded and become more acidic to the components within your vehicle.

When should I change my brake fluid?

Ideally brake fluid changes should be followed by your manufacturers handbook. However if you are introducing your vehicle to higher loads and high performance situations more often, this interval should drop. Say your handbook says service every 10,000 miles or 2 years. If you are not tracking your car but are modifying it then generally your brake fluid may still rely on the OEM factory information. You can still however cause excessive heat and moisture to develop in your brake fluid, which can cause failure.  You should therefore lower you service intervals down to maybe once a year or 8,000 miles. If you also then track your car and introduce it to further stress. Your intervals ideally should be below 5,000 miles. It doesn’t hurt to service your vehicle more often as brake fluid is relatively cheap. This can be little bit overkill but you should favour consistent braking over small maintenance costs anyways.

What will happen if I go over the recommended limit? You will run into problems with your brake fluid collecting moisture from the atmosphere due to heat and degregation. Brake fluid will lose its properties through heat and cold temperatures. This can be from the environment you car is used in, or from the amount of driving/track driving you do with it.  Brake fluid over a long term and over many heat cycles - loses its properties to lubricate,  protect and deal with the hydraulic pressure required. Meaning that if leave the brake fluid too long, the brake fluid may come out a completely different colour from what was put in. This is due to the fact that its lost its ability to act like brake fluid and has pretty much turned to water. The long term effects of this is increased damage and wear to all moving parts. This includes your ABS pump, brake lines, calipers, caliper seals and possibly also your master cylinder. Remember, servicing costs less than a brake failure, potentially leading too a crash!