Motorcycle Brakes:
How To Bleed Them

Basically there are 3 reasons why you want to bleed your motorcycle brakes:

  • After a big tumble the brake feels spongy and weak
  • You replaced the master cylinder kit
  • Your manual recommends that you replace brake fluid

My bike is upside down in regular intervals. So on more than one occasion I had the pleasure to find out the following: if the brake lever has been accidentally pulled in while the bike is restored to its normal rubber-side-down stance, the brake can feel spongy and weak.

A bubble of air has been introduced to the system. For a hydraulic brake system to work properly, all air must be removed and this process is called bleeding the motorcycle brakes.

What you will need

  • Usual hand tools
  • Catch bottle
  • Fluid specified in your manual
  • Clean environment

CATCH BOTTLE

Find yourself a piece of plastic pipe that fits exactly over the bleed nipple in the calliper. Then bore a hole in the cap of an old oil bottle and push the other end of the pipe through that.

Remember brake fluid strips paint very efficiently! If you do happen so spill some on a painted surface, wipe it off immediately and flush the area with hot water.

BRAKE FLUID

Only use the brake fluid specified in your manual or embossed on the reservoir cap of the master cylinder.

WORKING ON BRAKES

Absolute cleanliness is important when working around motorcycle brakes. Make sure no dirt can fall into the fluid reservoir.

Also double and triple check everything. A flailed motorcycle brake can put you into hospital quicker than you can blink! (Thanks Andy for testing that for us!)

Rear Brake

The rear brake is easiest to do. So let's start with this one

  • Put a ring spanner of the correct size over the nipple and check that the nipple can be opened and closed. If it is seized, take it to your dealer and let them sort it out.
  • Fit the nipple end of your catch bottle onto the nipple
  • Undo the reservoir top of the master cylinder which will either be a big nut, or a cap secured by two Philips head screws. If the screws are reluctant to budge, seat the screwdriver into its slot, then tap the end of the screwdriver with a hammer.
  • Check that the reservoir is full of fluid, topping up if necessary.
  • Put a cloth between the brake lever and your hand and pump the brake lever up and down about 5 times.
  • Holding the brake lever down, open the nipple with the spanner in your other hand, then close it again.
  • Repeat this whole operation about five times
  • Check the level of fluid in the reservoir, topping up when necessary. Continue until brake lever has good solid feel.
  • Lastly push brake pedal down for about 1 min. Make sure it does not move. If it moves down slowly under pressure it means that either the rubbers inside the master cylinder are worn or that there is leak at one of the fitting at either end of the brake pipe.
  • When you are happy close up the reservoir, ensuring that the expansion rubber is clean and pressed right back.
  • Remove the catch bottle and replace the rubber protector over the nipple. Clean up any brake fluid that may have got onto the disc or pads.

Front Brake

Exactly the same procedure is followed with some small differences:

  • You may have to loosen the throttle housing for clear access to the reservoir.
  • To get one hand onto the brake lever and one hand on the bleed nipple spanner will be more difficult.
  • There is more volume of fluid so it will take longer.
  • There are some front brakes that will just never bleed up properly. This is quite common on Brembo set-ups, less so on Nissin equipment

If your motorcycle brake is not coming right, read on before you lose your temper...

When a brake won't bleed up

When motorcycle brakes won't bleed, a bubble of air is moving down the brake pipe when you release the pressure at the nipple.

When you tighten up the bubble moves back up the pipe a bit and so never gets close to the nipple where it can be expelled from the system.

If you ask for advice on this problem people will tell you to reverse bleed by squirting brake fluid into the nipple with a syringe. This doesn't seem to work very well either.

So here is what Andy (my favourite mechanic suggests):

  • The brake nipple has to be the highest part of the system, so remove the calliper, brake pipe and master cylinder from the bike as a complete unit.
  • Using a trestle or something similar, clamp the master cylinder to a low point on your rig.
  • With a "G" clamp, clamp the calliper to the top of the rig with the bleed nipple uppermost.
  • IMPORTANT: Between the brake pads, slide a piece of wood or metal approximately the same thickness as the brake disc. If you don't do this you run the risk of popping the brake pistons right out of the calliper
  • Make sure that no part of the brake pipe is higher than the bleed nipple and start your pumping and opening and closing routine.

The brake should come right very quickly. If it doesn't, you have master cylinder rubber problems.


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