Question: My car has an automatic transmission with park,
so why do I need a parking brake?
Answer: The parking brake serves as a mechanical emergency
backup brake system. Should your hydraulic brakes fail, the parking
brake can be used to stop the vehicle.
Many people who have vehicles with automatic transmissions
rarely use their parking brake. They simply put the transmission
into park to lock the drive wheels when they park their vehicle.
Even so, it's important to use your parking brake periodically.
Using the parking brake regularly helps keep the cables freed
up so corrosion can't accumulate and cause binding. Applying
the parking brake also works the self-adjusters in the rear brakes,
which helps keep the linings in drum brakes properly adjusted
for minimum pedal travel. On cars with four wheel disc brakes
and locking rear calipers, using the parking brake keeps the threaded
self-adjusting mechanisms inside the rear caliper pistons working
freely to compensate for pad wear.
The parking brake system is fairly simple. On vehicles with
drum brakes in the rear, applying the parking brake pulls a pair
of cables that are attached to arms on the secondary brake shoes.
This forces both pairs of shoes outward against the drums to
lock the brakes. On four wheel disc brake applications, the disc
brake pads are pushed against the rotor by the caliper pistons.
This requires either a cam or screw mechanism inside the caliper
piston that pushes the piston out and holds it there, or a mini-drum
brake inside the rear rotor.
On rear disc brake applications with locking calipers, the
adjustment of the parking brake cable is especially important.
If the cable is adjusted too tight, there may not be sufficient
travel to work the self-adjusters and/or the brakes may drag.
If the cable is adjusted too loose, the parking brake may not
hold the vehicle. As a rule, most hand levers should travel only
about 4 or 5 "clicks" when properly adjusted.
On four-wheel disc brake systems that have "mini-drums"
inside the rear rotors, the parking brake works like a conventional
drum brake. Pulling on the cable forces the shoes outward against
the drum to lock the wheel. But unlike a full-sized drum brake,
there's no self-adjuster mechanism for the star wheel to compensate
for shoe wear because one isn't needed. The only time the parking
brake is applied is when the vehicle is at rest so shoe wear is
virtually nonexistent. The shoes should last the life of the
vehicle -- unless the parking brake is binding and causing them
to drag. The thickness of the shoe linings doesn't really matter
as long as there is enough lining left to hold the car on an incline
with normal cable travel.
On most vehicles, the left and right parking brake cables
are attached to a lever linkage called an "equalizer"
yoke under the vehicle. The equalizer yoke balances or equalizes
the amount of force that's applied to both cables when the parking
brake is applied. The equalizer linkage, in turn, is connected
to a single cable that runs to the parking brake lever or pedal.
An adjustment screw may be located on the front cable where it
connects to the equalizer, or where the cable attaches to the
parking brake lever.
Rust is the main concern with the brake cables and linkage.
Rust can cause the cables to bind in their sheaths, preventing
the brakes from being applied or released. If one cable freezes
up, the equalizer can't do its job so only one wheel will lock.
Though this may not create a problem when the parking brake is
used for parking, it could create a serious handling problem should
the system ever be called upon for emergency braking. The imbalance
would likely cause the one wheel to lock up and skid, possibly
throwing the vehicle out of control.
The equalizer linkage can also rust up, interfering with proper
application and release. Or it's hinge pivot can sometimes rust
or break loose rendering the parking brake useless. Lubricating
the pivot points and brake cables periodically with chassis or
brake grease can help protect against corrosion.
