Bleeding is a service procedure that involves purging air
from the master cylinder, brake lines, calipers, and wheel
If there are air bubbles in the fluid, they will compress
when the brakes are applied, causing either a low or soft
pedal. Bleeding gets the air out, leaving only
non-compressible brake fluid.
Air can enter lines when the system is opened for repairs.
Air can also enter the lines if the master cylinder
reservoir gets too low.
To remove air, bleeder screws on the calipers and/or wheel
cylinders are opened one at a time. Old fluid, along with
any air, is then drained, pumped, pushed or siphoned out
while fresh fluid is added to the master cylinder reservoir.
Manual bleeding is usually a two-person job. One person
pumps the brake pedal while another closes the bleeder screw
after each stroke to prevent air from being pulled back into
the system. Gravity bleeding doesn't involve any pumping.
The bleeder screws are opened and the fluid is allowed to
Gravity bleeding is slow and seldom used except on certain
import applications. Most professionals use power bleeding
because it is fast and does not require another person.
Compressed air is used to force new brake fluid through the
master cylinder to push out the old.
Another method sometimes used is vacuum bleeding. Special
equipment is used to siphon old fluid out through each
To get all the air out, brakes must be bled in the proper
sequence. Depending on how the hydraulics are split
(front/rear or diagonally), the usual sequence is to bleed
the wheels furthest from the master cylinder, then the
On most rear-wheel drive vehicles, the recommended sequence
is RR, LR, RF, LF. On front-wheel drive cars with diagonally
split brake systems, the sequence is RR, LF, LR, RF.
If the master cylinder is being replaced, it may have to be
bench bled before being installed.
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