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Best brake fluid? Castrol SRF vs Motul RBF 660

Updated May 2023 by Track Manual Team

Upgrading brake fluid is one of the first recommended things for prepping cars for the track and HPDE. Often the top brake fluid recommendations are either Castrol SRF or Motul RBF 660. Both are highly capable fluid for track days and HPDE but are they equal? In this article, we will explore the importance of brake fluid and the characteristics of both to help you make an informed decision.

Why brake fluid is important

Brake fluid is the core that powers a car’s braking system, being the hydraulic fluid that transfers power from pedal to pad. Let’s look at the braking system from a high level:

• When the driver presses the brake pedal, it applies that pressure to the brake fluid in the brake lines.
• The brake fluid is in the brake lines connected to each corner of the car, which then transfers that pressure to the caliper pistons.
• The caliper pistons compresses the brake pads to the rotors which reduces movement of the wheel, thus stopping the car.

If the brake fluid loses its ability to transfer this pressure, the car will lose its braking power and be unable to stop. Brake fluid is hygroscopic, meaning it absorbs moisture from the air. When it absorbs moisture, it loses its hydraulic ability hence causing the brake system to fail. A brake system is closed system, but it isn’t impervious from exposure to air whether through microscopic cracks or the brake fluid tank.

Different brake fluids perform differently and are often measured by their wet and dry boiling points.

What is wet boiling point?

Wet boiling is the boiling temperature of brake fluid when it has absorbed 3.7% of water content by volume. This measurement is in consideration for aged brake fluid over time as it absorbs water. With the frequency of changing brake fluid for motorsport prepped cars, this is less of a factor for high performance brake fluid.

What is dry boiling point?

Dry boiling point of brake fluid is the boiling temperature of new brake fluid straight out of a fresh bottle. This factor can be considered as the best performance metric, as the boiling point degrades over time as moisture is absorbed by the fluid.

So which should I use?

Let’s examine Castrol SRF vs Motul RBF 660:

*Data from Castrol and Motul

While Motul RBF 660 has an edge in a higher dry boiling point, Castrol SRF has a much larger advantage in wet boiling point which speaks to its longevity. Cost is fairly similar for the two. Both are performant fluids, but choice comes down to factors such as budget and frequency of change intervals. Frequent changes will help you take advantage of optimal performance, with intervals becoming shorter as you track more often.

You can purchase Castrol SRF here or Motul RBF 660 here.

While this article touches the surface on selecting the proper brake fluid for your car, it is only one of the components of your brake system. Do not forget to consider brake pads, brake lines, and other components to maintain top braking efficacy on the track.

You can read more about our other recommended products here: 🏆Top Products for the Track