Bike brake fluids

DOT or mineral oil: never mix them up

It is the one thing to check before ordering, and a mistake irreversible. The two fluids are incompatible: seals in a mineral-oil system swell and deteriorate on contact with DOT fluid, and vice versa. A brake filled with the wrong fluid needs replacing completely, not just bleeding.

• Mineral oil : Shimano, Magura, Campagnolo, Tektro and newer SRAM ranges that have switched to it. Usually red or blue, depending on the brand.
• DOT fluid : traditionally SRAM and Avid, as well as Hope, Hayes and Formula. Available as DOT 4 and DOT 5.1, which are compatible with each other.
• The brand printed on the lever or calliper tells you which to use. If in doubt, the brake manual will state it explicitly — do not rely on the colour of the fluid already inside.

Watch out for DOT 5

Despite its name, DOT 5 is not an upgraded version of DOT 5.1. It is a silicone-based fluid, incompatible with bicycle brake systems. Only DOT 4 and DOT 5.1 can be used in a bicycle brake designed for DOT fluid.

Why DOT brakes need bleeding more often

DOT fluid is hygroscopic : it absorbs moisture from the air over time. Dissolved water lowers its boiling point, and on a long descent the fluid can boil — the lever feels spongy and pulls back to the handlebar at the worst possible moment. Hence the recommendation for an annual bleed on DOT brakes, and opening a fresh bottle rather than using what is left in one opened two years ago. Mineral oil does not absorb water and needs bleeding less often, based on how the lever feels.

Viscosity and cold weather

Some brands offer a version low-viscosity of their mineral oil, which keeps the lever responsive in cold weather, when standard fluid thickens. A real advantage for anyone riding in the mountains or in winter.

Handling

The DOT fluid damages paint and irritates skin : wear gloves, protect the frame and wipe up any spills immediately. In either case, even a drop on a rotor or brake pad contaminates the brake — keep a disc brake cleaner to hand, and never reuse a soaked brake pad.

Beyond the fluid

The range also includes calliper service kits : pistons and seals, sold by model and piston diameter. These are the parts to order when a piston sticks or a calliper leaks, rather than replacing the entire brake.

To find out more: How to choose your disc brake fluid

See also: Brake tools · Bike cleaners and degreasers · Work stands