Bleeding the Brakes on a 2021 Ducati Monster 937

Shop parts for a 2021 Ducati Monster 937 Motorcycle.

Know Your Monster's Brake Layout Before You Start

The 2021 Monster 937 uses hydraulic disc brakes at both ends, and both circuits run through an ABS unit. Up front, twin discs are gripped by radial-mount calipers fed from a master cylinder on the right handlebar; out back, a single caliper is fed by a pedal-driven master cylinder with its own small reservoir. The two circuits are independent — this bike does not use a linked or combined braking system — so you bleed the front and the rear as separate jobs, each with its own fluid supply.

Because ABS is part of the plumbing, a normal flush through the caliper bleeders is still a straightforward garage task. What changes is the failure case: if the reservoir has ever run dry, or if the ABS unit or its lines have been disconnected, air can sit inside the pump where a lever-and-bleeder routine will not reach it. That situation calls for a dealer-level procedure, and we will come back to it.

When a Bleed Is Actually Needed

Brake fluid is hygroscopic, meaning it pulls moisture out of the air over time. Moisture lowers the fluid's boiling point, and a boiling fluid creates vapor that behaves exactly like air in the line — the lever goes long and soft right when you need braking most. That is why many riders refresh the fluid on a schedule even when nothing feels wrong.

Bleed sooner if any of these show up:

  • The front lever or rear pedal feels spongy, or the lever now travels closer to the grip than it used to.
  • The fluid in either reservoir looks dark, cloudy, or has visible debris.
  • You have just replaced lines, serviced a caliper, or had master cylinder work done — any of which lets air in.
  • The lever slowly sinks while you hold steady pressure at a stop.

Tools and Supplies to Gather

  • Fresh brake fluid of the type printed on the master cylinder reservoir cap, from a sealed container. Do not substitute a different type.
  • A box-end wrench that fits the bleeder valve snugly — a box-end grips better than an open-end and is far less likely to round the hex.
  • Clear tubing that fits tightly over the bleeder nipple, plus a catch bottle to hang it in.
  • A syringe or turkey baster for pulling old fluid out of the reservoir.
  • The correct driver for the reservoir cap screws.
  • Rags or shop towels, and water for rinsing spills.

A one-way check-valve bleeder or a vacuum bleeder makes this a much easier solo job, since you are not juggling the lever and the wrench at the same time.

Setting Up Without Damaging the Bike

Get the Monster upright and stable on a stand, on level ground. For the front circuit, turn the bars so the reservoir on the right side sits as level as possible before you open it — a tilted reservoir spills. Drape rags over the tank, the surrounding plastics, the dash, and the calipers. Brake fluid attacks paint and clear plastic on contact, so treat every drop as a problem: wipe it immediately and rinse the area with water rather than smearing it around.

Bleeding the Front Brake

  1. Remove the reservoir cap and diaphragm, then use the syringe to draw most of the old fluid out. Leave a little in the bottom so you do not push air into the master cylinder.
  2. Top up with fresh fluid from the sealed container.
  3. Push the clear tubing onto the front caliper bleeder and route the other end into the catch bottle, with the bottle positioned so the tube's end stays submerged.
  4. Squeeze the front lever and hold it. Do not release it yet.
  5. Open the bleeder about a quarter turn. Fluid and any trapped air will push out through the tube.
  6. Close the bleeder before you let the lever go. Releasing with the bleeder open pulls air straight back in.
  7. Repeat squeeze, hold, open, close, release until the fluid coming through the tube is clean and free of bubbles.
  8. If your bike has two front calipers, bleed each one in turn, working the caliper farthest from the master cylinder first and finishing with the closer one.
  9. Check the reservoir level after every few cycles and top it up. Letting it run low enough to draw air means starting over.

Bleeding the Rear Brake

  1. Open the rear reservoir, remove most of the old fluid, and refill with fresh fluid.
  2. Fit the tubing to the rear caliper bleeder and lead it to the catch bottle.
  3. Press and hold the pedal, crack the bleeder a quarter turn, then close it before releasing the pedal.
  4. Continue until the fluid runs clean and bubble-free, keeping the reservoir topped off throughout.

Finishing the Job

Tighten each bleeder snugly — firm, not forced; overtightening can damage the caliper threads. Fill each reservoir to the level marking, reinstall the diaphragm and cap, and wipe down every surface you touched. Then inspect the bleeder and the banjo fittings at the caliper and master cylinder for weeping, and leave a clean paper towel under each overnight if you want confirmation.

Testing Before You Ride

Both the lever and the pedal should feel firm and consistent, and neither should creep toward the end of its travel under steady pressure. Spin each wheel and listen for drag or a rubbing caliper. Then ride slowly in a safe, empty area and apply each brake separately before trusting them at speed.

If the lever will not firm up after several rounds, if it only firms up when you pump it, if bubbles keep appearing, or if you see fluid at the master cylinder or caliper, stop and diagnose rather than ride. And if the reservoir has run dry or the ABS unit has been opened, take the Monster to a shop — that air needs a diagnostic tool to clear properly.

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Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.