Bleeding the hydraulic brakes on an electric scooter: when, why, and how

Bleeding a hydraulic brake system involves removing air and old fluid from the circuit and replacing them with fresh fluid. On an electric scooter, this is necessary when the brake lever feels spongy or travels too far towards the handlebar, after any work on the brake lines, and regularly to replace fluid that degrades over time. The process requires a methodical approach and the correct type of fluid, but it is within the reach of a careful DIY enthusiast who has the appropriate kit for their brakes.

How does a hydraulic brake system work?

When you squeeze the lever, a small piston within the master cylinder on the handlebar pushes the fluid through the hose. The fluid is virtually incompressible; it transmits the pressure to the pistons in the caliper, which press the brake pads against the disc. Anything that is compressible within the system (such as air or a hose that expands) absorbs some of the lever travel, causing the brake to feel spongy.

That's why air is the enemy of hydraulic brakes, and why a hose that doesn't expand, like an aviation-grade hose, improves the feel. We explain this in our guide on aviation-grade brake hoses.

When is it necessary to bleed the brakes?

• Spongy lever: It feels soft and gives way before actually applying the brakes.
• Lever that touches the handlebar or whose point of contact changes from one braking action to the next.
• After intervention: Hose shortened or replaced, fitting changed, caliper removed from its circuit.
• After overheating: A long descent, a track session, resulting in a lever that became loose.
• Maintenance should be performed according to the schedule provided by the brake manufacturer. DOT-type fluids gradually absorb moisture from the air; their boiling point decreases over time, and a boiling fluid can cause air bubbles to form in the system during braking.

The right fluid, and nothing else

There are two main categories of hydraulic brake fluid for two-wheeled vehicles:

• DOT brake fluids (DOT 4, DOT 5.1) are glycol-based. They absorb moisture and can damage paint and certain plastics: wipe and rinse away any spills immediately
• Mineral oils, which are used by other brake manufacturers.

Brake seals are designed for specific types of brake fluid. Using the wrong fluid can cause the seals to swell or harden, potentially leading to brake failure – a serious mistake. The recommended brake fluid is usually indicated on the reservoir cap or the caliper; if in doubt, consult the manufacturer's manual. Also, be careful with "DOT 5" – it's a silicone-based fluid that is different from DOT 5.1 and is not a substitute for it.

The Hope Tech 4 V4 brakes that we install on our machines use DOT 5.1 fluid.

Here's the step-by-step method:

Each manufacturer has its own procedure and kit (syringes, fittings, bleed block): follow its manual for the details specific to your brake system. The general principle remains the same:

1. Preparation: Ensure the scooter is stable, and rotate the handlebars so that the reservoir of the brake lever is horizontal and at its highest point. Protect the disc brake, paint, and floor with cloths.
2. Remove the wheel or pads, and place a bleed block between the pistons to prevent them from retracting.
3. Open the reservoir on the lever and install the collection container or syringe provided in the kit.
4. Connect the new fluid syringe to the bleeder valve on the caliper (the small hollow screw designed for this purpose), then open the bleeder valve.
5. Slowly push the fluid from the caliper towards the lever. The air will naturally rise with it; continue until the fluid coming out at the lever is clean and free of bubbles.
6. Close the bleed valve, tap the hose and caliper to dislodge any remaining air bubbles, then gently actuate the lever to force the air bubbles back up to the reservoir.
7. Fill the level and close the reservoir without trapping any air, then carefully clean away any traces of liquid.
8. Clean the brake pads, reattach the wheel, and pump the lever until you feel a firm contact point.

After the bleeding process: the checks

• The lever should be firm, with a clear and consistent point of contact that doesn't shift when squeezed.
• Firmly squeeze the lever and hold it: it should not slowly depress. If it does, there is air trapped in the system or a seal is leaking.
• Check for leaks at the fittings, the bleed valve, and the reservoir.
• Perform a test at low speed before resuming a normal pace.

If the brake pads have come into contact with liquid, they will no longer brake correctly. Replace them, as explained in our guide on brake pads.