Brake Fluid

Your guide to buying correct brake fluid or changing fluid or for your car and saving yourself a huge repair bill.

Vehicle brakes use hydraulic brake systems that use brake fluid to transfer the pressure of the braking action from the drivers pedal or lever to the master cylinder and then on to the caliper and brake pads. Sounds simple right?

In fact it is far from that, vehicle brake hydraulics systems are a carefully balanced and precision system that needs respect for cleanliness when servicing and most important the RIGHT type of brake fluid to be used.

There are two basic groups of brake fluid, Glycol fluids and silicone fluids (there are also one or two cars in France using mineral fluids but we leave that aside for the purpose of this article).

Glycol fluids are the most common and used in 99.9% of motor vehicles in various grades. They are named by their DOT (Department of Transport coding) either DOT 3,4 or 5. The higher the letter the higher the brake fluid quality.

Most cars run happily on DOT 3 brake fluid which is a basic fluid that has a low water content. Yes shocked you may be but water is in all Glycol fluids and the scary part is that being hygroscopic the fluids actually ingest water over years of exposure to the elements and water content over 10 years can be as high as 14%. Think about what that does for corrosion inside your brake system and how that reduces the effectiveness of your brakes and promotes brake fade as the water vaporise in extreme brake use when the caliper gets hot.

DOT 4 is basically the same fluid with more water refined out from the brake fluid. DOT 4 will give you a slightly firmer pedal but note, if you leave a can of DOT 4 fluid standing once the seal is broken within a few weeks it will be DOT 3 or worse. For this reason always use brake fluid from a sealed container and buy it in small bottles NOT gallon drums. Some Companies like EBC Brakes refuse to sell brake fluid in anything more than a top up bottle and quite rightly so . As a performance brake supplier selling brake fluid in gallons opens up a whole barrel of worms.

Within the Glycol fluid range some manufacturers have used their own coding because some bright spark years ago started applying the DOT5 code to glycol fluids when the world regard DOT 5 as being a Silicone fluid. One such blend of highly refined brake fluid known as BF307 is worth a look and is a highly refined glycol for sports cars and race use and has a 307 degree boiling point.

Boiling point is important for performance driving, the higher the boiling point the better but it is also worth noting that if any brake caliper itself gets above 250 degrees in temperature you have a problem, not even a Formula one car uses heat paint indicators above 250 degrees C.

Glycol fluids draw water in through the hoses in your vehicle which you will be amazed to hear are also to a tiny degree porous. That’s why it is wise for drivers to flush and replace brake fluid every 5-6 years to refresh those tired old brakes.

There are several other things to note about Glycol brake fluids. First they are toxic so never drink or ingest them and secondly they are very caustic so when working with Glycol brake fluids keep them fluid and contaminated rags or your fingers away from vehicle paintwork. Wash off any spills quickly with soapy water.

Silicone Brake Fluids are another story, they are not toxic (although it is never good sense to drink any such fluids) and not caustic. For this reason some classic car and motorcycle builders use silicon fluids to avoid paintwork and plastic damage. Harley Davidson for example use silicone fluids in some bikes.

Always check the spec of brake fluid on your vehicle master cylinder or in the manufacturers handbook.

Mixing glycol and silicone fluids is definitely not advised, it causes an interface and globulates which means the pressure transfer does not happen effectively and certain caliper and cylinder hydraulic seals which may be viton or nitrile rubber are designed for one type of fluid and are destroyed by the other.

Once you have flushed and changed your brake fluid you will need to bleed the brakes and informaton on how to do this may be found on this link.