AI quick summary
- SRAM uses DOT 4/5.1 (glycol-based, absorbs water); Shimano uses mineral oil (petroleum-based, does not). Never mix the two — a single drop destroys seals.
- A spongy lever that firms up after a few pumps means air in the line; a firm lever with weak power at the caliper means contaminated pads.
- Bleed from the caliper upward for SRAM (syringe at the bleed port) and from the lever downward for Shimano (funnel at the lever) — follow the maker's direction.
- Heat from long descents can boil old DOT fluid; replace fluid every 1–2 years for mountain use, every 2–3 for road.
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Why hydraulic brakes need bleeding
Hydraulic brakes work by incompressible fluid transmitting force from lever to caliper. Air — even a few bubbles — is compressible, which is why a brake with air in the line feels spongy and the lever can be pulled to the bar. Heat makes it worse, because the air and any water absorbed into the fluid expand. DOT fluid (used by SRAM) is hygroscopic and absorbs water over time, lowering its boiling point from ~230 °C fresh to under 150 °C in two years. Mineral oil (Shimano) is not hygroscopic, but it still picks up contaminants that soften the lever.
DOT vs mineral oil — the two hydraulic ecosystems
| Property | DOT 4 / 5.1 (SRAM) | Mineral oil (Shimano) |
|---|---|---|
| Base chemistry | Glycol ether, hygroscopic | Petroleum, non-hygroscopic |
| Boiling point, fresh | ~230 °C | ~210 °C |
| Water absorption | Yes, lowers boiling point | No |
| Seal compatibility | Specific EPDM seals | Different EPDM |
| Interval | 1–2 years | 2–3 years |
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Symptoms: air, contamination, or both
A lever that firms up after three pumps has air in the line. Bleed it. A lever that is firm but the brake still does not stop has contaminated pads — oil or DOT fluid on the rotor has glazed the pads. Sanding the pad surface with 220-grit and cleaning the rotor with isopropyl alcohol restores bite if the contamination is light; severe contamination means new pads.
If only one brake is spongy, you have a leak — inspect hose fittings and the caliper piston seals before bleeding, or you will repeat the job in a week.
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SRAM (DOT) bleed: bottom-up with syringes
Remove the wheel and pads (DOT strips pads). Insert a bleed block in the caliper. Attach a syringe of fresh DOT 4 (or 5.1 for higher-spec) to the caliper bleed port, and an empty syringe to the lever bleed port. Push fluid from the caliper syringe up through the system and out the lever, tapping the hose to dislodge bubbles. When the lever syringe runs clean with no bubbles, close the lever port, then push a final small volume from the caliper to set pressure, and close the caliper port. Clean everything with isopropyl — DOT strips paint.
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Shimano (mineral oil) bleed: top-down with a funnel
Shimano's standard method uses a funnel at the lever and a syringe at the caliper. Remove the wheel, insert the block, attach the funnel to the lever. Push mineral oil from the caliper syringe upward until it flows clear into the funnel, flicking the hose to free bubbles. Squeeze the lever a few times to actuate pistons, then push more oil. Replace the funnel cap, top off, and remove.
Shimano also specifies a 'short' lever-only bleed that refreshes the master cylinder without disturbing the caliper — useful if you have to open the system to shorten a hose. For a full service, do the top-down bleed; for a quick top-off after a hose trim, the short bleed is enough.
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Common traps and post-bleed checks
Use the manufacturer's fluid, always. 'Compatible' substitutes swell seals over months. Keep all containers closed — DOT absorbs humidity from air in minutes. Never reuse bled fluid. If you have to bleed more than once to get a firm lever, you have air still trapped — typically at the caliper piston seals or the lever's master cylinder. Rotate the lever on the bar so the bleed port is the highest point before the final push.
After reassembly, pump the lever to set the pistons. Bed in pads with 30 slow stops from a moderate speed before any long descent. A properly bled hydraulic brake should lock a wheel with two fingers at the lever's first third of travel, with a firm, wooden feel — never spongy.
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