There are two types of brake fluid used in hydraulic bicycle bike brakes today:
- DOT Fluid and;
- Mineral Oil
Certainly the most commonly used brake fluid in use today, due to its wide use in the automotive industry, is DOT fluid. All DOT fluids (with the exception of DOT 5) are made up of a poly-glycol base.
Glycol-based fluids consist of a mixture of ingredients with as many as ten separate substances making up the final product. These substances can be broken down into four key components:
- A lubricant, such as polythene or polypropylene, to keep parts moving freely - 20-40%.
- A solvent diluent, usually glycol ether, which determines the fluid's boiling point and viscosity and accounts for 50-80% of the fluid.
- A modifier-coupler, which changes the amount of swelling of exposed rubber parts.
- Inhibitors, to prevent corrosion and oxidisation.
DOT brake fluid is required to meet strict standards and specifications set out by the Society of Automotive Engineers and the Department of Transportation (DOT) - hence the name. These standards centre around maintaining brake fluid performance in a range of temperatures (high and low) and also specify the minimum boiling temperatures which the fluid manufacturers must adhere to.
Unlike DOT brake fluid, Mineral Oil brake fluids are not governed by any standards or regulatory body, therefore the technical information on the various substances that contribute to their make-up is usually hard to come by. The likes of Shimano and Magura have no doubt spent a lot of time and money refining their proprietary Mineral Oil brake fluids so you can understand why there is a degree of secrecy on the subject.
You can find out more about DOT and Mineral Oil brake fluid here.