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Why do brakes need to be bled?

Why do brakes need to be bled?

Why do brakes need to be bled?

All hydraulic brakes are bled or purged for one of two reasons:

  • To remove air bubbles from the brake system, or
  • To replace the brake fluid with new.


Removing Air


bleeding air at quad master cylinder


For hydraulic systems to work efficiently they must rely on an in compressible fluid as a means of transferring forces.

In hydraulic braking systems the input forces you create by operating the brake lever travel via the brake fluid to operate the caliper pistons, which in turn, move the brake pads to contact the rotor. This is only possible because the brake fluid is in-compressible.

Think of brake fluid then as a means of communication. The brake fluid transmits the message given at the master cylinder (lever) to the slave cylinder (caliper) with minimal loss in quality.

Air in the system causes a breakdown in this communication. Input forces created by operating the brake lever can no longer be transmitted effectively as these forces are wasted as the air inside the system is compressed.


Replacing the Brake Fluid


time for a change


Part of the general maintenance and upkeep of your hydraulic brake is to periodically change the brake fluid. Over time your brake fluid will become old and discoloured as it takes on contaminates from the environment. Dirt, air and water can and will find their way in eventually.

Replacing the brake fluid occasionally will keep these to a minimum and extend the life of your brake and keep it performing like new.

Alex Mansell Epic Bleed Solutions

Alex Mansell, Head honcho
Shares tips and advice on the dark-art of brake-bleeding. Rides bikes when time permits.
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