Prepare for ASE Brakes (A5) certification test content area “E” (Miscellaneous Systems Diagnosis and Repair).
Describe what is required of a parking brake.
Describe the parts and operation of the parking brake as used on a rear drum brake system.
OBJECTIVES: After studying Chapter 78, the reader should be able to: Continued
Describe how a parking brake functions when the vehicle is equipped with rear disc brakes.
Explain how to adjust a parking brake properly.
OBJECTIVES: After studying Chapter 78, the reader should be able to:
application cables control cables electric parking brake (EPB) • equalizer intermediate lever red brake warning lamp vacuum servo
Before 1967, most vehicles had only a single master cylinder operating all four brakes. If fluid leaked at just one wheel, the operation of all brakes was lost. This required a separate method to stop the vehicle in case of an emergency. After 1967, federal regulations required use of dual or tandem master cylinders where half of the braking system has its own separate hydraulic system. In case one-half of the system fails, a dash brake warning lamp lets the driver know that a failure has occurred. The term parking brake has replaced the term emergency brake since the change to dual master cylinder design.
According to Federal Motor Vehicle Safety Standard (FMVSS) 135, the parking brake must hold a fully loaded (laden) vehicle stationary on a slope of 20% up or down grade. The hand force required cannot exceed 80 lb. (18 N) or a foot force greater than 100 lb. (22 N). See Figure 78–1 for a typical parking brake system.
Figure 78–1 Typical parking brake cable system showing the foot-operated parking brake lever and cable routing. Continued
PEDALS, LEVERS AND HANDLES
Parking brakes are applied by a pedal, a lever, or a handle from inside the vehicle.
Continued Foot pedals and floor-mounted levers are the common means of applying parking brakes. See Figure 78–3.
Figure 78–3 Typical hand-operated parking brake. Note that the adjustment for the cable is underneath the vehicle at the equalizer.
All parking brakes are applied manually and the release procedure varies with the design of the parking brake control.
Figure 78–4 A ratchet mechanism is used to lock parking brakes in the applied position.
All parking brake controls incorporate a ratchet mechanism to lock the brake in the applied position.
Continued 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.”
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. See Figure 78–5.
Figure 78–5 A remote-mounted parking brake release lever. Continued
Figure 78–6 Automatic parking brake release mechanisms usually use a vacuum servo to operate the release lever.
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.
A customer called and asked a dealer for help because the parking brake could not be released. The service technician discovered that the customer was attempting to release the parking brake by depressing the parking brake pedal, as was done on the customer’s previous vehicle. The service technician pulled on the release lever and the parking brake was released.
Pump To Release A metal rod connects the vacuum servo to the upper end of the parking brake release lever. When the engine is running (to provide vacuum) and the shifter is placed in gear, an electrical contact closes to energize the solenoid and route vacuum to the servo. The servo diaphragm then retracts the rod, which releases the parking brake. See Figure 78–7. Continued
Figure 78–7 The two plastic vacuum tubes on the steering column are used to release the parking brake when the gear selector is moved from park into a drive gear.
PARKING BRAKE WARNING LAMP
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 LINKAGES
Parking brake linkages transmit force from the pedal, lever, or handle inside the vehicle to the brake friction assemblies.
Continued Linkage Rods Parking brake linkage rods made from solid steel are commonly used with floor-mounted actuating levers to span the short distance to an intermediate lever or an equalizer. Linkage Cables Parking brake cable is made of woven-steel wire encased in a reinforced rubber or plastic housing. The housing is fixed in position at both ends, and is routed under the vehicle through mounting brackets that hold the cable in position, yet allow a small amount of movement. The cable slides back and forth inside the housing to transmit application force.
The ends of parking brake cables are fitted with a wide variety of connectors that attach to actuating devices, other linkage parts, or the wheel friction assemblies. Control cables attach to the parking brake pedal, lever, or handle inside the vehicle, and transmit force to an intermediate lever or equalizer to the application cables . The application cables use the force passed through the linkage to apply the friction assemblies. See Figure 78–8.
Figure 78–8 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. Continued
Always inspect parking brake cables for proper operation. A cable that is larger in diameter in one section indicates that it is rusting inside and has swollen. A rusting parking brake cable can keep the rear brake applied even though the parking brake lever has been released. This can cause dragging brakes, reduced fuel economy, and possible vehicle damage due to overheated brakes.
Look for Swollen Parking Brake Cables Figure 78–9 Notice how rust inside the covering of this parking brake cable has caused the cable to swell.
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. See Figure 78–10.
Figure 78–10 Intermediate levers in the parking brake linkage increase the application force.
Parking brake pedals, floor-mounted levers, & pivoting under dash handles are types of levers to increase parking brake application force.
Straight-pull parking brake handles are not levers; they are commonly connected to other levers in the linkage. 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. Continued
Linkage Equalizers In some parking brake linkages, the rods or cables to the two friction assemblies are adjusted separately. If the adjustments are unequal, one brake will apply before the other, preventing full lining-to-drum contact at the opposite wheel and greatly reducing the holding power of the parking brake.
Continued To prevent unequal application, most parking brake linkages use an equalizer to balance the force from the parking brake control, and transmit an equal amount to each friction assembly. See Figure 78–11.
Figure 78–11 A cable guide is a common type of parking brake linkage equalizer. Equalizers come in many shapes and sizes, but the simplest is the cable guide attached to a threaded rod. This type of equalizer pivots or allows the inner cable to slide back and forth to even out the application force. Continued
Figure 78–12 Some parking brake equalizers are installed in the brake cable.
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.
Figure 78–13 Many parking brake linkages use both an intermediate lever and an equalizer.
Linkage Design The number of different parking brake linkage designs is almost as great as the number of models on the road.
Most combine intermediate levers and equalizers in various ways and use from one to four cables to actuate the friction assemblies. Continued
The parking brake standard requires that the vehicle be held stationary on a 20% grade facing either uphill or downhill. Many drum parking brake systems attach the parking brake lever on the secondary (rearward) shoe and push the primary (forward facing) brake shoe against the drum. The parking brake cable enters the backing plate from the front of the vehicle (front entry). Because the primary shoe is attached to the secondary shoe on dual-servo brakes, any forward motion of the vehicle tends to wedge the primary shoe into the brake drum and force the rear secondary lining also against the drum.
FRONT AND REAR ENTRY PARKING BRAKE CABLES Continued
Applying only the forward brake shoe tends to hold the vehicle best when the vehicle is being held on a hill with the front pointing downward. To help provide the same holding power for a vehicle being held from backing up, some vehicles reverse the parking brake arrangement for the right side. Instead of having the parking brake cable enter the backing plate from the front, this style has the cable entering from the rear (rear entry). In this case the right rear brake has the parking brake lever installed on the primary shoe.
DRUM PARKING BRAKES
The most common types on vehicles and light trucks.
Continued Figure 78–14 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. Excellent parking brakes because of high static coefficient of friction and self-energizing action. Integral Drum Parking Brakes Mechanically apply the rear drum service brakes to serve as parking brakes. Shown at right.
Integral drum parking brakes are the most common type because of superiority in this application, and because it is simple and inexpensive to design a parking brake linkage into a drum brake. The typical integral drum parking brake has a pivoting lever mounted on one brake shoe, and a strut placed between the lever and the other shoe. The strut may be fitted with a spring that takes up slack to prevent noise when the parking brake is not applied. The end of the lever opposite the pivot is moved by the parking brake cable, which enters through an opening in the backing plate. All integral drum parking brakes operate in essentially the same manner. See Figure 78–15.
Figure 78–15 The parking brake cable pulls on the parking brake lever, which in turn forces the brake shoe against the drum. Continued
Figure 78–16 The inside “hat” of the disc brake rotor is the friction surface for the parking brake shoes.
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.
Continued Inside the drum is a small dual-servo drum brake friction assembly that serves as the parking brake. The rotor splash shield, or a special mounting bracket, provides the backing plate for the friction assembly.
Rear disc auxiliary drum parking brakes use the dual-servo friction assembly design because it provides the most holding power, and does so equally in both forward and reverse directions. Dual-servo parking brake friction assemblies operate in essentially the same manner as service brakes except that the wheel cylinder is eliminated and the friction assembly is actuated mechanically. See Figure 78–17.
Continued NOTE: T o reassemble a drum brake is to remember the parking brake lever is usually attached to the secondary (rearward) brake shoe. The parking brake strut attaches between the shoes with the spring toward the front of the vehicle (remember, “ spring forward ”).
Figure 78–17 A typical rear disc brake auxiliary drum brake friction assembly. Continued
CALIPER-ACTUATED DISC PARKING BRAKES
Caliper-actuated disc parking brakes are used on vehicles with rear disc brakes equipped with floating or sliding brake calipers. The single-piston construction of these calipers makes them easier to mechanically actuate than multiple-piston fixed calipers. In this design, a special mechanism in the caliper applies the caliper piston mechanically. The mechanism is operated by a parking brake cable attached to a lever that protrudes from the inboard side of the caliper.
Figure 78–18 A Ford rear brake caliper ball and ramp-type apply mechanism.
Ball and Ramp Actuation The ball and ramp actuating system found in Ford rear brake calipers has three steel balls located in ramp-shaped detents between two plates.
One plate has a thrust screw attached that is threaded into an adjuster mechanism in the caliper piston. The other plate is part of the operating shaft that extends out of the caliper. The actuating lever is mounted to the end of this shaft. As the parking brake cable moves the lever and rotates the operating shaft, the balls ride up the ramps and force the two plates apart. The operating shaft plate cannot move because it butts against the caliper body. The thrust screw plate, which is pinned to the caliper body to prevent it from rotating, is driven away from the operating shaft and toward the rotor where the thrust screw moves the caliper piston to apply the brake. See Figure 78–19.
Figure 78–19 Operation of a ball and ramp-type rear disc brake caliper parking brake. Continued
Figure 78–20 Automatic adjustment of a ball and ramp-type rear disc brake parking brake occurs when the service brakes are applied.
Adjustment of the ball and ramp linkage in the caliper is automatic, and takes place during service brake application. When the caliper piston moves away from the thrust screw, an adjuster nut inside the piston rotates on the thrust screw to take up slack created by wear.
Continued A drive ring on the nut prevents it from rotating in the opposite direction when the parking brake is applied.
Screw, Nut, and Cone Actuation GMs’ rear disc parking brake uses a screw, nut, and cone mechanism to apply the caliper piston.
Figure 78–21 A typical GM rear disc brake with an integral parking brake. This type uses a screw, nut, and cone mechanism to apply the caliper piston. Continued
Figure 78–22 Parking brake application of a General Motors rear drive brake caliper.
The actuator screw with parking brake lever attached extends through the caliper body.
Continued The caliper piston has a specially shaped nut that threads onto the actuator screw, butts against the backside of the cone, and is splined so it can’t rotate unless the cone does. The cable moves the lever and rotates the actuator screw. The cone is a slip fit in the piston, free to rotate unless held against a clutch surface located near the outer end of the piston bore.
Figure 78–23 Automatic adjustment of a General Motors rear disc brake caliper.
The nut unthreads along the screw, and jams the cone against the clutch surface of the caliper piston.
This prevents the cone from rotating as the caliper piston is keyed to the brake pad, which is fixed in the caliper. Movement of the nut along the actuator thread forces the cone and piston out against the inboard pad to apply the brake. Adjustment of the screw, nut, and cone occurs automatically during normal operation as the service brakes are released. Continued
Note that the outer end of the nut is in constant contact with the cone whenever the service brakes are not applied. If the automatic adjusting system fails, tension of the adjustor spring against the thrust bearing at the back of the piston will retract the cone and piston from the rotor until the cone does contact the nut, resulting in low brake pedal.
Figure 78–24 Removing the piston from a typical General Motors rear disc brake caliper. Continued See Figures 78–25 through 78–31.
Figure 78–25 Installing the piston into a General Motors rear disc brake caliper. Continued
Figure 78–26 A piston installation tool is required to fully install the piston into a General Motors rear disc brake caliper. Continued
Figure 78–27 A spanner wrench (or needle-nose pliers) can be used to rotate the caliper piston prior to installing the disc brake pads. A notch on the piston must line up with a tab on the back of the brake pad to keep the piston from rotating when the parking brake is applied. Continued Figure 78–28 After removing parking brake lever and thrust bearing, remove the antirotation pin.
Figure 78–30 To test the piston adjuster, thread the thrust screw into the piston. Hold the piston and pull the thrust screw outward 1/4 in. (6 mm). The adjuster nut should not turn when the thrust screw retracts. Replace the piston assembly if not functioning correctly. Figure 78–29 Unscrew the thrust screw from the piston with an Allen (hex) wrench. After removing the thrust screw, push the piston out of the caliper bore. Continued
Figure 78–31 To adjust the parking brake cable on a Ford vehicle equipped with rear disc brakes, start by loosening the cable adjustment until the cables to the calipers are slack Tighten until the caliper lever moves. Position a 1/4-in. drill bit or dowel into the caliper alignment hole. Adjustment is correct if the parking brake lever does not hit the 1/4-in. dowel. Continued
When diagnosing any brake problem, apply the parking brake and count the “clicks.” This method works for both hand- and foot-operated parking brakes. Most vehicle manufacturers specify a maximum of 10 clicks. If the parking brake travel exceeds this amount, the rear brakes may be worn or out of adjustment.
The Parking Brake Click Test If the rear brake lining is usable, check for the proper operation of the self-adjustment mechanism. If the rear brakes are out of adjustment, the service brake pedal will also be low. This 10-click test is a fast and easy way to determine if the problem is due to rear brakes. CAUTION: Do not adjust the parking brake cable until the rear brakes have been thoroughly inspected and adjusted.
PARKING BRAKE CABLE 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.
Continued General procedure for parking brake adjustment:
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.
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.
Figure 78–32 After checking that rear brakes are OK and properly adjusted, parking brake cable can be adjusted. Always follow manufacturer’s recommended procedure. Continued See Figure 78–33.
Figure 78–33 Many hand-operated parking brakes are adjusted inside the vehicle.
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.
Continued 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.
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. Figure 78–34 Always check that the brake shoes contact the anchor pin.
To prevent possible parking brake cable adjustment problems when installing new rear brakes, always observe the following:
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.
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.
Figure 78–35 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. Continued
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 Figure 78–36 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.
Electric parking brake ( EPB ) systems are available using two different designs including:
How Does An Electric Parking Brake Work? Some vehicles use an electric parking brake (EPB) that can be activated when the vehicles stops and then goes off as soon as the gas pedal is pressed, preventing the car from moving while stopped.
A cable-pulling type that uses an electric motor to pull the parking brake cable rather than a mechanical handle or foot pedal.
A more advanced unit uses a computer-controlled motor attached to the brake caliper to activate it.
Figure 78–37 An electric parking brake button on the center console of a Jaguar.
Government regulation requires that the parking brake be able to hold a fully loaded vehicle on a 20% grade.
The typical parking brake uses either a hand-operated lever or a foot operated pedal to activate the parking brake.
On a typical drum brake system, the parking brake cable moves a parking brake lever attached to the secondary brake shoe. The primary shoe is applied through force being transferred through the strut.
All parking brake cables should move freely. The rear brakes should be adjusted properly before the parking brake is adjusted.