If you are a track day enthusiast, you know how important it is to have a reliable and high-performance brake fluid. Brake fluid is the medium that transfers the hydraulic pressure from the brake pedal to the brake calipers, and it must withstand high temperatures and pressures without boiling or losing its effectiveness. Boiling brake fluid can cause a spongy pedal feel, reduced braking power, and even complete brake failure.
There are many brake fluids on the market, but two of the most popular and recommended ones for track use are Motul RBF 700 and Castrol SRF. Both of these fluids are DOT 4 rated, which means they meet the minimum standards set by the Department of Transportation for brake fluid performance.
However, they also exceed these standards by a large margin, offering exceptional dry and wet boiling points, stability, and compressibility.
But which one is better for your track car? In this article, we will compare Motul RBF 700 and Castrol SRF in terms of their specifications, performance, price, and availability. We will also give you some tips on how to choose the best brake fluid for your needs and how to bleed your brakes properly.
Price and Availability
Brake fluid is not a one-time purchase, but a consumable item that needs to be replaced regularly. Therefore, the cost and availability of the fluid can affect your budget and convenience.
Motul RBF 700 and Castrol SRF are both premium brake fluids that are not cheap. However, they also offer premium performance that can save you money in the long run by preventing brake fade, wear, and damage. The price of both fluids may vary depending on the retailer, location, and demand.
However, as of May 2023, the average price of Motul RBF 700 is about $30 per 500ml bottle, while the average price of Castrol SRF is about $57 per liter bottle. This means that Motul RBF 700 is cheaper per volume than Castrol SRF, but it also has a smaller bottle size. However their prices are subject to change. You can view the updated price of Motul RBF here and Castor SRF here.
Motul RBF 700 and Castrol SRF are both widely available online and offline. You can find them at most automotive parts stores, online retailers, and track day vendors. However, they may not be as easy to find as some of the more common brake fluids, such as Motul RBF 600 or ATE TYP 200. Therefore, you may want to stock up on your preferred fluid before heading to the track or order it online in advance.
Specifications
Let’s start by looking at the technical specifications of Motul RBF 700 and Castrol SRF. These are the numbers that indicate how well the brake fluid can resist boiling, maintain its viscosity, and compress under pressure. Here is a table that summarizes the main specs of both fluids:
| Brake Fluid | Dry Boiling Point (°C) | Wet Boiling Point (°C) | Viscosity at -40°C (cSt) | Viscosity at 100°C (cSt) | Compressibility at 100°C (10^-5/bar) |
|---|---|---|---|---|---|
| Motul RBF 700 | 336 | 223 | 1400 | 2.5 | 5.6 |
| Castrol SRF | 310 | 270 | 1202 | 3.5 | 6.5 |
As you can see, Motul RBF 700 has a higher dry boiling point than Castrol SRF, which means it can withstand higher temperatures before boiling. However, Castrol SRF has a higher wet boiling point, which means it can retain its performance better after absorbing moisture from the air.
Moisture is inevitable in brake fluid over time, and it lowers the boiling point of the fluid. Therefore, a higher wet boiling point indicates a longer service life of the fluid.
Motul RBF 700 also has a lower viscosity than Castrol SRF at both low and high temperatures, which means it flows more easily through the brake system and offers a more consistent pedal feel.
However, Castrol SRF has a lower compressibility than Motul RBF 700 at high temperatures, which means it does not expand as much under pressure and offers a more precise brake modulation.
Performance
While specifications are important, they do not tell the whole story of how a brake fluid performs on the track. There are other factors that affect the performance of a brake fluid, such as its chemical stability, corrosion resistance, lubricity, compatibility, and seal swell.
These factors are harder to measure and compare objectively, but they can make a difference in how well the brake fluid protects your brake system and maintains its effectiveness.
Motul RBF 700 and Castrol SRF are both synthetic brake fluids that are formulated with high-quality ingredients and additives that enhance their performance and durability.
They both have excellent chemical stability, meaning they do not degrade or change their properties over time or under extreme conditions. They both have good corrosion resistance, meaning they do not cause rust or damage to the metal components of your brake system.
They both have good lubricity, meaning they reduce friction and wear between moving parts of your brake system. They both have good compatibility, meaning they do not react negatively with other brake fluids or materials in your brake system. And they both have good seal swell, meaning they do not cause excessive expansion or contraction of the rubber seals in your brake system.
The main difference between Motul RBF 700 and Castrol SRF in terms of performance is their boiling point behavior. Motul RBF 700 has a higher initial boiling point, but it also drops more rapidly after absorbing moisture. Castrol SRF has a lower initial boiling point, but it also retains it more steadily after absorbing moisture.
This means that Motul RBF 700 may offer better performance in the short term, but it also requires more frequent bleeding and replacement. Castrol SRF may offer better performance in the long term, but it also requires more careful monitoring and maintenance.
The best way to compare the performance of Motul RBF 700 and Castrol SRF is to test them on your own car and track conditions. Depending on your driving style, brake system, and ambient temperature, you may find one fluid to suit your needs better than the other.
However, both fluids are proven to offer excellent performance and reliability for track use, and you can’t go wrong with either one.
Conclusion
Motul RBF 700 and Castrol SRF are both excellent brake fluids for track use. They both offer exceptional boiling points, stability, and compressibility that can handle the high temperatures and pressures of hard braking. They both have high-quality synthetic formulas that protect your brake system from corrosion, wear, and damage. And they both have wide availability and reasonable prices for their performance level.
The main difference between Motul RBF 700 and Castrol SRF is their boiling point behavior. Motul RBF 700 has a higher dry boiling point but a lower wet boiling point than Castrol SRF.
This means that Motul RBF 700 may perform better initially but require more frequent bleeding and replacement than Castrol SRF. Castrol SRF has a lower dry boiling point but a higher wet boiling point than Motul RBF 700. This means that Castrol SRF may perform better over time but require more careful monitoring and maintenance than Motul RBF 700.
The best way to choose between Motul RBF 700 and Castrol SRF is to test them on your own car and track conditions. Depending on your driving style, brake system, and ambient temperature, you may prefer one fluid over the other. However, both fluids are proven to offer excellent performance and reliability for track use, and you can’t go wrong with either one.
| Feature | Motul RBF 700 | Castrol SRF |
|---|---|---|
| DOT rating | DOT 5.1 | DOT 4 |
| Dry boiling point | 325°C (617°F) | 310°C (590°F) |
| Wet boiling point | 216°C (420°F) | 205°C (401°F) |
| Viscosity | 1.6 cSt @ 20°C | 1.77 cSt @ 20°C |
| Color | Amber | Amber |
| Price | $26.99 (1 liter) | $59.99 (1 liter) |
| Lifespan | 18 months | 12 months |