Providing quality brake repairs depends largely on whether all parts
are free to move as designed and assembly tolerances are adhered to.
Disk brakes are subject to high levels of rust and corrosion because
disk brake parts are constantly exposed to the elements. Rust and corrosion
affects two main areas of the disc brake system.
Rusts Impact on Brake Pad Wear
The first area that is effected is the caliper and brake pads
ability to move freely. Corrosion prevents the free movement of the
caliper housing or brake pads causing premature pad wear. If the caliper
housing is not free to move on its mounting pins the outboard brake
pad will not release completely. Rust and corrosion in the caliper casting
holes is a leading cause of this problem. In the casting holes the rust
grows causing the inner diameter of the casting hole to
shrink. This causes the rubber bushing or boots to be squeezed
tightly around the caliper mounting pins and restricting free caliper
movement. Rust in the brake pad mounting points causes premature inboard
pad failure because it prevents free movement of the pads. The only
way to restore free movement is to remove the rust buildup or replace
Rusts impact on pedal pulsation
The second area impacted by rust and corrosion is the mating surface
between the hub and rotor. Over time the hub and rotor surfaces corrode.
This corrosion causes the rotor not to fit correctly on the hub. The
rust results in what is commonly referred to as jacking.
It pushes the rotor away from the hub mounting surface causing lateral
runout. This runout is a leading cause of brake comebacks. It causes
a condition known as disk thickness variation or DTV which is often
referred to as parallelism. DTV is the cause of most pulsation problems.
The inner and outer friction surfaces of a rotor must be parallel to
one another with no more than .0005 variation. Excessive runout
causes the high spots on the rotor to wear as they pass between the
brake pads. The two wear spots occur 180 degrees opposite one another.
The result is a rotor with alternating thick/thin spots which cause
the pulsation felt in the brake pedal.
Explanation of runout
Runout is defined as the lateral or side to side movement of the rotor
as it rotates. When the rotor turns, the thick and thin spots pass between
the brake pads. As the brakes are applied the caliper piston must travel
out and in when the thick/thin spots pass between the brake pads. This
caliper piston movement results in hydraulic fluid movement which is
felt at the brake pedal in the form of pedal pulsation.
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