Hughesy's Tech Tip - Choosing Brake Fluids Penrite Blog Post

Posted on 16 Feb 2017

What do you think is most important when choosing a brake fluid?

Most drivers today probably don’t have much of an idea of what brand or type of brake fluid they have in the vehicle. Some may be concerned with the boiling temperature of their brake fluid (such as race drivers who only use their cars at the race circuit) and others if it meets the DOT specification as per their owners or service manual. 

Everyday commuters who frequently drive their vehicles on the streets, may need to be more concerned if their fluid meets current DOT specification. Brake fluids that do not meet current DOT specifications can speed up the deterioration of brake components and can also lead to the malfunctioning of systems such as ABS during cold weather. Most people have little understanding of Brake Fluids, so here are some Q & A’s about it –

What is DOT specification?

DOT is the abbreviation for the 'Department of Transportation', which is a USA government transportation department. The DOT set standards such as FMVSS (Federal Motor Vehicle Safety Standard) which is very similar to the JIS in Japan or DIN in Germany. The following table shows the DOT brake fluid specifications:




Dry Boiling Temp

Wet Boiling Temp

Viscosity (100°)

Viscosity (-40°)

Ph. Value



over 205°

over 140°

over 1.5cst

under 1500cst




over 230°

over 155°


under 1800cst


DOT 5.1


over 260°

over 180°

over 1.5cst

under 900cst




over 260°

over 180°

over 1.5cst

under 900cst



Dry Boiling Temp: Boiling point when the fluid is brand new, no moisture absorption.

Wet Boiling Temp: Boiling point with fluid that has 3.7% water by volume. After 1-2 years of fluid use.

Viscosity : a measure to represent the brake fluid flow property. Higher the value, the more difficult for the fluid to flow. If the value is high when the air temperature is low, the fluid can have a negative effect on ABS performance.

pH Value : value to show acidity / basicity of a solution. If the pH value is lower than 7.0 (strong acidity), the fluid can accelerate corrosion of other brake components

What is boiling point temperature?

The temperature at which the fluid boils. Water boils at 100° whereas brake fluid with high boiling point will boil at over 300°C, and low boiling point brake fluid will boil around 140°C depending on condition of fluid..

Why is a low boiling point temperature not acceptable?

When driving a vehicle, brake pad temperature can reach extremely high temperatures (up to 300 °C). This heat gets passed onto the brake fluid through the  brake calipers, which can raise the fluid temperature over 200°C. If the brake fluid is repeatedly heated past it's boiling point, some of the fluid vaporizes and creates air bubbles within the brake lines. This can lead to a very dangerous situation where the brake lines are carrying air instead of brake fluid.  Air can be compressed which is why it is bled out of brake systems so that the fluid can compress the pistons inside the brake calipers, applying the brakes.

What is percent water by volume?

The most common ingredient of brake fluid is glycol-ether. This fluid is hygroscopic which mean it absorbs moisture from the atmosphere. The 'percent by volume' is a measure of the water content in the brake fluid.

The higher the DOT number, the higher the brake fluid performance ?

This is not exactly correct. The DOT numbers categorizes the fluids by various uses.

Specification       Application use

DOT 3                                    Cars with small to medium sized engines

DOT 4                                    Cars with larger sized engine and/or for use in competition

DOT 5.1                 Cars with larger sized engine and/or for use in competition and or in Cold Climate regions

DOT 5                    Silicone, Special application cars and motorcycles (Hummer, Harley-Davidson)

What are the major differences between types of brake fluids?

DOT 3 versus DOT 4           Boiling point temperature

DOT 4 versus DOT 5.1       Boiling point temperature & viscosity at low temperature

DOT 5 versus DOT 5.1,      DOT 5 is Silicone Fluid and not compatible with DOT 5.1, DOT 4 or DOT 3 fluids
DOT 4, DOT 3


How frequently should brake fluid be replaced?

For everyday drivers, who use brake fluid that exceed DOT 4 specification, fluid replacement once every 2 years is acceptable. For people who use DOT 3 fluid, fluid replacement every year is recommended. For people who drive aggressively on winding roads, fluid replacement every six month to a year is recommended. For people who drive their cars on the race circuit, replacement before each event is recommended.


Is it safe to mix old and new fluid ? Or, mix different grades of  fluid ?

This is not recommended. By adding new brake fluid to old brake fluid, you are mixing water absorbed brake fluid with new product lessening the effect and lowering the lifespan of the newer fluid. If both products are glycol based DOT approved fluids, the mixing is possible with no major functional problems. Complete replacement of the fluid at the correct life span is the safest option.


What is Mineral Brake Fluid ?

Mineral brake fluid are products NOT made from Glycol-Ether. DOT fluids are nearly all synthetic products. Mineral based brake fluids as used by Rolls Royce in a few of its models during the 1960’s to 1980’s are suspension and brake fluids that have different properties to DOT fluids.


Can I put Silicone Brake Fluid in my vehicle?

This is normally not recommended, especially when the vehicle has been set up to run with normal DOT type brake fluid. Silicone fluid could cause brake failure in a system designed to run on DOT fluid. Alternatively, systems designed to be used with Silicone Brake Fluid, should not have DOT fluid placed into them. Although Silicone and DOT fluids can be interchangeable, the systems need to be completely flushed before swapping. Silicone             Brake fluid does not absorb water which makes it a good choice for Veteran Vintage and Classic models not driven regularly.


Further details on these products are available on their respective product information sheets found on the Penrite web site:

Penrite recommend “The Right Oil for the Right Application”

Click Here to visit the Penrite Recommendation Guide, which will ensure you receive the correct oil for your application

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