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Mineral oil vs DOT brake fluid: which one your hydraulic brakes need

Hydraulic bike brakes use one of two fluid families that must never be mixed. This page explains which fluid your brakes take, how to recognise air in the circuit and when to bleed.

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Answers are written by a language model solely from this base's documents, with numbered sources. They can be wrong and aren't legal, medical or financial advice: check the sources before any important decision.

Two incompatible fluid families

Mineral oil (paraffin based) is used by Shimano, Magura and Tektro. DOT fluid (polyethylene glycol based) is used by SRAM, Avid, Hope, Hayes and Formula. SRAM, Avid and Hope take DOT 5.1; Hayes and Formula take DOT 4.

The seals are made for one fluid type. DOT in a Shimano brake makes the seals swell and disintegrate within 24 to 48 hours, and mineral oil in a SRAM brake causes the same failure. On a Shimano system the damage is irreversible and means replacing both the lever and the calliper.

DOT 4 and DOT 5.1 are compatible with each other, and 5.1 can be used in a system designed for 4. DOT 5, which is silicone based, is a different fluid and must never be used.

Signs that your brakes need bleeding

The guides list five signs: a spongy lever, abnormally long lever travel, inconsistent braking from one pull to the next, a small clicking noise in the lever when squeezing slowly, and bubbles visible in the reservoir.

A lever that firms up after pumping a few times is the classic sign of air in the circuit. Before changing pads or the calliper, start with a bleed. A low fluid level, a stuck piston or, more rarely, a worn master cylinder are the other causes to check, in that order.

Bleeding a Shimano brake

Mount the bike with the lever horizontal so the bleed port is the highest point, and remove the wheel and the pads: a single drop of mineral oil ruins them.

Push fresh mineral oil slowly from the calliper up to the lever until it comes out clear with no bubbles, squeeze the lever to dislodge trapped air, then close up and wipe any residue, as mineral oil attacks paint.

Frequently asked questions

How often should hydraulic brakes be bled?

The Shimano guide recommends at least every 12 months, every 6 months for enduro or DH riding, and sooner if the lever softens. Another guide suggests 12 to 18 months for leisure riding once or twice a week and 6 to 9 months for intensive mountain biking.

Which fluid do Shimano road brakes use?

Mineral oil. The guides list Shimano Deore, SLX, XT, XTR, 105 and Ultegra as mineral oil systems, along with Magura MT5, MT7 and MT8.

Why bleed with the lever horizontal?

Bleeding with the lever at an angle traps bubbles in the hose bends. A horizontal lever puts the bleed port at the highest point of the circuit, where air can escape.

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