Brake Fluid Types and Differences

Brake fluid has gone through several evolutions in the last ten or so years with new formulas, materials, and standards being introduced. We have swept away the clutter and this page should help you understand each type of fluid and why it exists.

First, the golden rule of brake fluid: change it often. And change it more often if it's a track car. Nearly all brake fluid on the market is hygroscopic and will absorb any moisture. The higher water content the thicker the fluid and the more likely it is to boil under extreme heat. So it's vital that you change your fluid regularly. Some brands require fluid changes more often than others so pay attention to the advice on the label. No brake fluid should be in use for longer than two years. Once opened the shelf life of a can is generally considered to be no more than one year.

The key to a good brake fluid is how much it can resist compression when pressure is applied. The more the fluid compresses the longer the pedal travel and the longer it takes for the fluid to push on the caliper pistons. Fresh brake fluid out of the bottle is very resistant to compression but as it absorbs moisture and/or air it will compress more and more (water and air have very poor resistance to compression).

All brake fluid can be divided into two types: glycol hygroscopic and silicone hydrophobic.

GLYCOL-BASED WATER ABSORBING (HYGROSCOPIC). The vast majority of brake fluid will absorb moisture to keep it from interfering with the braking system. Not only does water compress a lot but it can also rust or even freeze in the internal metal brake lines and calipers. And it's a poor lubricant for sliding caliper pistons. Also, as water boils it creates a gas that can block fluid from circulating. This is the number one reason why brake fluid should be changed often (at least every two years for a normal street car). The boiling point of brake fluid is used as the metric for evaluating its per