Question: I feel a pulsation or vibration in my brake
pedal every time I stop. But the brakes seem to work fine. Is
anything wrong?
Answer: A pulsating brake pedal, which may be accompanied
by a shuddering or jerky stop during normal braking, usually means
a warped rotor or an out-of-round drum -- although it can sometimes
be caused by loose wheel bearings, a bent axle shaft or loose
brake parts. If the vehicle is equipped with ABS, however, some
pedal feedback and noise is normal during panic stops or when
braking on wet or slick surfaces. But you should not experience
any ABS pedal feedback when braking normally on dry pavement.
The faces of a disc brake rotor must be parallel (within .0005
inch on most cars) and flat (no more than about .002 to .005 inches
of runout) otherwise it will kick the brake pads in and out when
the brakes are applied, producing a pulsation or vibration that
can be felt in the brake pedal as the rotor alternately grabs
and slips.
You can often see warpage in a brake rotor by simply looking at
it. If the rotor has telltale glazed or discolored patches on
its face, chances are it is warped. Measuring it with a dial indicator
and checking it for flatness with a straight edge will confirm
the diagnosis.
Resurfacing the rotor to restore the faces will usually eliminate
the pulsation (unless the rotor is bent or is badly worn and has
started to collapse in which case the rotor must be replaced).
But it may only do so temporarily because of metallurgical changes
that take place in the rotor. Hard spots often extend below the
surface of the rotor. Resurfacing will restore the surface, but
the hard spot may reappear again in a few thousand miles as the
rotor wears. For this reason, GM and others recommend replacing
warped rotors rather than resurfacing them.
Pedal pulsation caused by drum warpage isn't as common, but it
can happen. A drum can sometimes be warped out-of-round by applying
the parking brake when the brakes are hot. As the drum cools,
the force of the shoes causes the drum to distort.
What causes a rotor to warp? Overtorquing or unevenly torquing
the lug nuts with an impact wrench is a common cause. For this
reason, most experts recommend using a torque wrench to tighten
lug nuts when changing a wheel. There are also special torque-limiting
extension sockets called "Torque Sticks" that can be
safely used with an impact wrench to accurately tighten lug nuts.
But a plain impact wrench should never be used for the final tightening
of the lug nuts because most provide no control whatsoever over
the amount of torque applied to the nuts.
Overheating can also cause rotors to warp. Overheating may be
the result of severe abuse or dragging brakes. Defects in the
rotor casting, such as thick and thin areas can also cause uneven
cooling that leads to warpage. Hard spots in the metal due to
casting impurities can be yet another cause.
