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Parking Brake Systems (Drum Brakes) 6.1.9

An overview of drum brake parking brake systems
by

Ivan W. Anderson

on 7 May 2013

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Transcript of Parking Brake Systems (Drum Brakes) 6.1.9

Linkage Levers The rods and cables transmit application force in direct proportion. If 50 lbs of application force is delivered to one end of a rod or cable, 50 lbs of force will be available at the other end as well.

Unfortunately, the amount of physical force a driver can apply to the parking brake control is insufficient for effective parking brake operation. For this reason, all parking brake linkages contain one or more levers that increase application force.

The cable from the activating lever to the equalizer is commonly called the control cable. From the equalizer, the individual brake cables are often called application cables. These individual cables can usually be purchased separately. Parking Brake Pedals A parking brake pedal is applied by depressing it with a foot. The ratchet engages automatically and the pedal remains in the depressed position.

The pedal is released by a pull or a small T-handle or lever under the dash. This disengages the ratchet mechanism, and allows a return spring to move the pedal to the unapplied position.

On some vehicles, the release lever is integrated into the underside of the dash and connects to the release mechanism through a rod or cable.

Both brake shoes should make contact with the anchor pin
at the top. If not, check the parking brake cable for improper adjustment or improper installation of the brake shoes.

Feel the tension of the parking brake cable under the vehicle. It should be slightly loose (with the parking brake “off”).

Lubricate the parking brake cable to ensure that water or ice will not cause rust or freezing of the cable. This is necessary because even though the parking brake lever is released inside the vehicle, a stuck parking brake cable could cause the linings to remain out against the drums. Continued To prevent possible parking brake cable adjustment problems when installing new rear brakes, always observe the following: The parking brake cable pulls on the parking brake lever, which in turn forces the brake shoe against the drum. Continued Typical parking brake cable system showing the foot-operated parking brake lever and cable routing. If the parking brake needs to be adjusted (will not hold on a hill or requires excessive lever movement), always check and adjust the rear brake adjustment before adjusting the parking brake cable. A 1/8-in. (3-mm) drill bit is placed through an access hole in the backing plate to adjust this General Motors leading-trailing rear parking brake. Adjust the parking brake cable until the drill can just fit between the shoe web and the parking brake lever. Always check that the brake shoes contact the anchor pin. NOTE: The rear parking brake adjustment should always be checked whenever replacing the rear brake linings. It may be necessary to loosen the parking brake cable adjustment to allow clearance to get the drum over the new linings. This could happen because someone may have adjusted the parking brake cable during the life of the rear linings. After checking that rear brakes are OK and properly adjusted, parking brake cable can be adjusted. Always follow manufacturer’s recommended procedure. Adjust the cable at the equalizer (equalizes one cable’s force to both rear brakes) if necessary until there is a slight drag on both rear brakes. A typical rear disc brake auxiliary drum brake friction assembly. Another type of equalizer installs in a long application cable that runs from the linkage at the front of the vehicle to one rear brake. Some parking brake equalizers are installed in the brake cable. A lever in the parking brake linkage under the vehicle is called an intermediate lever, and provides leverage in addition to that supplied by the parking brake control. Straight-pull parking brake handles are not levers; they are commonly connected to other levers in the linkage. Parking brake pedals, floor-mounted levers, & pivoting under dash handles are types of levers to increase parking brake application force. Intermediate levers in the parking brake linkage increase the application force. Automatic Parking Brake Release Some vehicles with pedal-operated parking brakes have an automatic release mechanism that disengages the parking brake using a vacuum servo controlled by an electrical solenoid. Automatic parking brake release mechanisms usually use a vacuum servo to
operate the release lever. All parking brakes are applied manually and the release procedure varies with the design of the parking brake control. Typical hand-operated parking brake. Note that the adjustment for the cable is underneath the vehicle at the equalizer. NOTE: Some vehicles are equipped with an automatic adjusting parking
brake lever/cable. Simply cycling the parking brake on/off/on three times
is often all that is required to adjust the parking brake cable. Release the parking brake. Both rear brakes should be free and not dragging. Repair or replace rusted cables or readjust as necessary to ensure that the brakes are not dragging. Many hand-operated parking brakes are adjusted inside the vehicle. Make certain that the rear service brakes are adjusted correctly and the lining is serviceable.

With the drums installed, apply the parking brake 3 or 4 clicks. There should be a slight drag on both rear wheels. General procedure for parking brake adjustment: Most manufacturers specify a minimum of 3 or 4, and a maximum of 8 to 10, clicks when applying the parking brake. Most vehicle manufacturers specify that the rear brakes be inspected and adjusted correctly before attempting to adjust the parking brake cable. Always follow the manufacturer’s procedure exactly. PARKING BRAKE CABLE ADJUSTMENT This type of equalizer pivots
or allows the inner cable to slide back and forth to even out the application force. Equalizers come in many shapes and sizes, but the simplest is the cable guide attached to a threaded rod.
A cable guide is a common type of parking brake linkage equalizer. Whenever the parking brake is engaged, a red brake warning lamp lights on the dash. On most vehicles, this is the same lamp that lights when there is a hydraulic or brake fluid level problem.

The lamp for the parking brake warns the driver that the parking brake is applied or partially applied. The warning helps prevent damage or overheating to the brake drums and linings that could occur if the vehicle was driven with the parking brake applied.

If the red BRAKE warning lamp is on, check the parking brake
to see if it is fully released. If the BRAKE lamp is still on, the parking brake switch may be defective, out of adjustment, or
there may be a hydraulic problem. PARKING BRAKE WARNING LAMP Some vehicles were equipped with a system that required the driver to depress the parking brake pedal to release the parking brake once it was set. The rubber pad on the parking brake pedal usually states “push to release.” All parking brake controls incorporate a ratchet mechanism to lock the brake in the applied position. A ratchet mechanism is used to lock parking brakes in the applied position. Foot pedals and floor-mounted levers are the common means of applying parking brakes. Parking brakes are applied by a pedal, a lever, or a handle from inside the vehicle. PEDALS, LEVERS AND HANDLES Many parking brake cables can be removed easily from the backing plate using a
1/2-in. (13-mm) box-end wrench. The wrench fits over the retainer finger on the end of the parking brake cable. It is often difficult to remove a parking brake cable from the backing plate due to the design of the retainer. The many fingers used to hold the cable to the backing plate can be squeezed all at once if a hose clamp is used to compress the fingers. A wrench as shown here can also be used. The Hose Clamp or Wrench Trick The rotor splash shield, or a special mounting bracket, provides the backing plate for the friction assembly. Rear Disc Auxiliary Drum Parking Brakes Rear disc service brakes with fixed calipers commonly have a parking brake drum formed into the hub of the brake rotor. Inside the drum is a small dual-servo drum brake friction assembly that serves as the parking brake. The inside “hat” of the disc brake rotor is the friction surface for the parking brake shoes. Integral Drum Parking Brakes
Mechanically apply the rear drum service brakes to serve as parking brakes. Shown at right. Excellent parking brakes because
of high static coefficient of friction and self-energizing action. Notice the spring at the end of the parking brake strut. This antirattle spring keeps tension on the strut. The parking brake lever is usually attached with a pin and spring (wavy)
washer and retained by a horseshoe clip. Continued The most common types on vehicles and light trucks. DRUM PARKING BRAKES Parking Brakes (Drum Systems) 6.1.9
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