Disc brake systems for cars and light trucks have been standard
equipment for over a decade. By design, all disc brakes are self
adjusting and therefore, they require no periodic readjusting to
compensate for brake shoe wear. However, certain malfunctions may
cause a disc brake to drag. This drag will reduce mileage and result
in premature brake pad and rotor wear. Disc brakes are used primarily
on front wheels but are also used on the rear of some vehicles.
It is critical for both safety and maximum fuel economy that disc
brakes operate properly. It is important to note that any brake
system on a given vehicle operates in the proper front to rear sequence
and that the left and right sides operate in unison to avoid undue
premature lockup or lateral forces.
What causes disc brakes to drag?
Several malfunctions are possible. On the floating caliper, single
piston design, the most likely culprits are a build-up of dirt,
corrosion, or loss of the protective lubrication on the guide pins,
sleeves or ways. These prevent the caliper from fully retracting
when the brake pedal is released.
The four piston, non-floating design is used only on larger pick-up
trucks and vans. Four piston types tend not to retract as a result
of corrosion build up in the caliper hydraulic. Small incremental
increases in corrosion inside the caliper bores prevent the pistons
from fully retracting after each brake application. If one or more
pistons fail to fully retract, the shoe will drag the rotor resulting
in excessive drag and reduced mileage.
How are these problems corrected?
If external corrosion, dirt, or lack of lubrication is the problem,
the standard procedure is disassembly in accordance with manufacturer's
shop manual procedures, cleaning of all parts with appropriate solvents,
inspection for wear, metal fatigue, and failure, lubrication using
the recommended lubricant and reassembly. If brake drag results
from a piston or pistons not fully retracting, then caliper disassembly,
inspection, and rebuilding will be necessary. Observation of the
manufacturer's cautions in the shop manual, noting the recommended
bleed down procedures for hydraulic systems in anti-lock braking
systems and hydro-boost type systems, is essential.
Summary
Disc brake system failures are infrequent. Disc brakes can and do
drag. Obvious indications of disc brake drag are:
- Excessive wear of one brake pad (Note: it is normal for the
inner pad of a single piston, floating type disc brake system
to have a slightly higher wear rate than the outer pad.)
- Vehicle pull - If all alignments are within specification,
a car or truck can still pull to one side if a disc brake drags.
- Tapered pad wear - This failure, most common on four piston,
fixed caliper type, is caused by one or more pistons failing to
fully retract.
