Does your scooter's brake lever feel spongy and does it almost reach the handle? Then there is air in the hydraulic system. Air is compressible, brake fluid is not, so your braking power is lost in the stroke of the lever. Bleeding removes that air and immediately refreshes the fluid.
When to bleed
- Spongy lever — the pressure comes late and feels springy; after quick pumping it feels firmer for a short while.
- System opened — a new brake hose, brake caliper or brake master cylinder, or brake pads where the line has been disconnected.
- Fluid older than about two years — glycol attracts moisture, the boiling point drops and with hot braking vapour forms: the same spongy feel. Check the workshop manual for the interval.
- Dark fluid — fresh is light straw yellow to clear; brown or cloudy means it needs changing.
If the lever slowly sinks while you hold it squeezed, that is not air but a leak.
Which brake fluid
On scooters and mopeds with a hydraulic disc brake, DOT 4 is common. What your brake needs is almost always printed on the reservoir cap — read it before you buy. DOT 3, DOT 4 and DOT 5.1 are glycol-based and can be mixed with each other; DOT 5 is silicone-based and must never be mixed with them. Mineral brake fluid has no DOT class and belongs to a different brake system.
Use an unopened bottle: an opened bottle has already absorbed moisture. Glycol attacks paint and plastic, so cover the handlebar area and rinse off any spilt fluid straight away.
What you need
- Ring spanner or socket wrench for the bleed nipple — 8, 10 or 11 mm is common; measure your own nipple.
- Clear hose that fits tightly around the nipple, plus a catch bottle.
- Fresh brake fluid in the prescribed class.
- Screwdriver, cloths, water, gloves and eye protection.
Step by step: the lever method
- Put the scooter on its centre stand with the handlebar straight, so the tank is level. Place cloths underneath.
- Unscrew the lid, remove the rubber membrane, and fill to the max line. If you are changing the fluid, first suck out the old fluid with a syringe.
- Slide the spanner over the nipple before you push the hose onto it. The other end goes into the bottle, into a little fluid at the bottom.
- Squeeze the lever in three to five times and hold it in.
- Open the nipple a quarter turn. The lever travels towards the grip and you will see fluid with air bubbles flowing out.
- Tighten the nipple before you release the lever. If you release it first, the caliper draws air back in.
- Release the lever and repeat step 4 through 6.
- Top up the reservoir after every two or three pumps. Never let it run dry, otherwise you will draw air into the brake master cylinder.
- You are done once only clear fluid without bubbles comes out and the lever feels firm.
- Tighten the nipple moderately; the thread is in aluminium and the tightening torque is specified in the workshop manual. Put on the dust cap, refill the reservoir, and replace the diaphragm and cover.
If the lever stays spongy
Air bubbles tend to linger in bends and at high points. While pumping, gently tap the brake caliper and along the line with the handle of a screwdriver. If that doesn't help, the air is at the highest point: the master cylinder at the lever. Briefly loosen the banjo bolt there with the lever squeezed, and immediately tighten it again with a generous cloth underneath. A well-known trick: secure the lever against the handle overnight with a cable tie.
A vacuum bleeder draws at the nipple, so you don't have to pump. Watch for air leaking in along the thread; a wrap of PTFE tape usually solves that.
Classic mistakes
- Leave the nipple open while you release the lever.
- Let the reservoir run dry.
- Topping up with a different grade than the cap specifies.
- Reuse fluid from an old, opened bottle.
Test afterwards
The lever must feel firm and stop at the same point upon repeated squeezing. The wheel must not rub and the brake light must switch. Brake at walking speed on your own property first and check after the first ride whether the nipple and banjo bolts are dry.
The right parts straight away
You need brake fluid; the brake bleeders are listed separately, and you replace a damaged nipple with a separate bleed nipple. If air keeps getting in, check the brake lines, brake calipers and brake levers, and filter on the left by your brand and model. If you do end up opening the brake, take fresh brake pads and, if necessary, a brake disc with you. Further reading: replacing brake pads, replacing the throttle cable and brake cable for cable brakes, scooter maintenance, getting your scooter ready for winter and the FAQ.
