Bleeding the Brakes
Bleeding is a service procedure that involves purging air from the
master cylinder, brake lines, calipers, and wheel cylinders.
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 dribble out.
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 bleeder screw.
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 closest wheels.
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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