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How we reviewed this product: we studied the manufacturer's specifications, materials, included parts and warranty terms, read owner feedback where available, and compared it with similar products. We did not run our own lab tests.
Castrol React SRF is one of the most respected brake fluids in motorsport, used in categories from sports cars to superbikes. Its standout feature is an unusually high wet boiling point, which means it holds its performance even after absorbing moisture.
Quick verdict
- Best for: racing, rallying and serious track-day cars.
- Type: silicone-ester based (not a DOT 5 silicone fluid).
- Strengths: very high dry and wet boiling points, resists vapor lock, firm pedal.
- Watch out for: more than any normal road car needs; must be flushed in fully, not just topped up.
What it is
SRF uses silicone-ester technology. Retailers list a dry boiling point of about 310 to 320 °C and a wet boiling point of about 270 °C (518 °F), much higher than most racing fluids. It is less hygroscopic than typical glycol fluids, meaning it absorbs less water. Its compressibility and viscosity are similar to DOT 3 and DOT 4 fluids, so it bleeds like a normal fluid and gives a firm "hard pedal" feel.
Key features in plain English
- Exceptional wet boiling point: stays effective longer between changes.
- Vapor lock resistance: designed to keep a high vapor lock point through its service life.
- Seal compatible: works with seals and metals used in normal polyglycol brake systems.
- Not DOT 5 silicone: despite the silicone-ester base, it is not the silicone DOT 5 type.
Pros and cons
Pros
- Among the highest wet boiling points available
- Firm, consistent pedal under heavy heat
- Trusted at the top levels of motorsport
Cons
- Overkill for daily driving
- Topping up with normal fluid reduces its benefits, so flush fully
- Not for systems that require mineral oil
How to switch to SRF
Drain and flush the old fluid out completely with SRF, then top up only with SRF. A pressure bleeder such as the Motive Power Bleeder makes a full flush much easier.
FAQ
Is SRF the same as DOT 5?
No. DOT 5 is silicone-based and must not be mixed with glycol fluids. SRF is a silicone-ester fluid that is compatible with standard polyglycol systems.
Do I need SRF for a fast road car?
Usually not. A high-quality DOT 4 such as ATE TYP 200 is plenty for most street use.
Before you buy brake fluid
- Check your owner's manual or the reservoir cap for the fluid type your vehicle needs.
- Never use DOT 5 (silicone) fluid unless your system is designed for it, and never mix it with glycol fluids.
- Only open a new, sealed bottle. Brake fluid absorbs moisture from the air once opened.
- Brake fluid damages paint. Wipe up spills right away.
Safety note: brakes are safety parts. If you are not sure how to do the job, have a qualified mechanic do it.