If you ever "open" the brake system, the entire brake system needs to be bled.
Replacing the brake master cylinder, any brake line, or and of the brake calipers
would require an entire brake system bleed.
Rule of thumb: alway bleed the caliper farthest away from the brake master
cylinder first. Work you way from the farthest corner to the nearest corner
in succession. If you have an ABS unit, the ABS unit is bled last. When
doing a full system brake bleed, we like to have at least two large (quarts) of
fresh brake fluid available.
Start by jacking up the corner to be bled first. If you have
enough jack stands, jack the entire car up and remove all wheels and tires; this
makes accessing the brake calipers a lot quicker. If you cannot jack the
entire car up, you'll need to do each corner/wheel one at a time.
Once the wheel is off the hub, find the brake caliper and access the brake bleed
valve. Keep in mind that the rear calipers have two brake bleed
valves per brake caliper; we like to bleed the top bleed valve first as our main
bleed valve, and then bleed the lower one after a few times just to purge all
the old fluid out. It's cleaner to run a short length of hose attached to
the brake bleed valve and loosen with an 8mm box wrench; the box wrench is used
to minimize rounding the hex fastener. The hose runs to a container that
will collect the old fluid. Now, if you have the stock brake bleed valves,
you'll need a helper to effectively bleed the brakes properly. Have your
helper pump the brake pedal 2 to 3 times and then hold. While he holds the
brake pedal down, release the pressure through the brake bleed valve by
loosening the 8mm brake bleed valve. Once pressure is immediately release,
close the brake bleed valve. Keep an eye on the brake master cylinder, as
you need to keep refilling the brake master reservior with fresh brake fluid
throughout the entire procedure. Repeat the pump, pump, pump, hold, open
valve, close valve procedure until you see fresh, clean brake fluid coming
through. Repeat for all four corners, farthest to closest to the brake
master cylinder (and then to the ABS unit if you have one).
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