How would you check the air parking brake?
Start the engine and wait until the air system reaches system pressure
Puh in the yellow-colored diamond-shaped dash control valve knob to release the park brakes.
It should stay in; if it doesn’t, check for a pressure loss or a defective control valve.
Test the park brakes by putting the truck in low gear, moving it forward at full speed, and applying the park brakes by pulling out on the knobs.
The truck should stop promptly.
What affects the park brake performance?
Park brake performance is affected by brake free stroke adjustment. An excessive free stroke will reduce both park and service brake performance.
What does the governor manage?
Compressor loaded and unloaded cycles. It defines the system pressure.
What values is system pressure in most trucks set at?
System pressure in most trucks is set at values between
120 and 135 pSI, with 125 psi being most typical.
What is cut out pressure?
Cur out pressure is the pressure at which the governor outputs the unloader signal to the compressor.
The unloader signal is maintained until the pressure in the supply tank drops to the cut-in valve.
What is one way to easily check the governor’s operation?
One method is to drop the aiW top of the governor, release the lock-nut and turn the adjusting screw either clockwise to lower or counterclockwise to raise the cut-out pressure.
What happens if the unloader signal not delivered to the compressor unloader assembly?
Will result in high system pressures.
What is usually an indication of governor or compressor unloader malfunction?
If the safety pop-off valve on the supply tank trips (this ours at 150 psi)/
What happens when the compressor unloader cycles?
It triggers the purge valve in the air dryer sump, ejecting moisture and any accumulated sludge.
The purge valve may become clogged and need to be cleaned.
What temperature should the air drier heater operate?
The heater should operate when the temp falls below about 40 degrees F.
How should you test the purge valve?
By cycling the compressor through several cut-in/cut-out cycles, and observing the operation of the purge valve. Each time the unloader cycles, there should be a discharge of air and moisture from the bottom of the air dryer.
How do you check the air system for leaks brakes applied?
- First block the wheels on the vehicle.
- Build air pressure to governor cut-out, release the parking brakes and shut off the engine.
- Walk around the unit, listening for audible air leaks.
- Then check dash gauges for system pressure loss.,
- Investigate any out-of-specification loss.
According to the Commercial Vehicle Safety Alliance, under what conditions shall a vehicle be declared out of service?
A vehicle with more than 20% of its air brakes defective (measured with the engine off, reservoir pressure at 80-90 psi, and the brakes fully applied.
How do you check the air system for leaks brakes released?
- Begin by starting the engine and bringing the air supply to system pressure.
2, With the engine idling, make and hold a full brake application until pressure stabilizes.
If the compressor is unable to maintain reservoir pressure of 89-90 psi with the engine at idle and the brakes fully applied, it is recommended that the truck be deadlined until repairs are completed.
How do you examine the truck for audile air leaks?
Shut off the engine, and have an assistant hold the brakes applied while you examine the truck for audible leaks.
What part of the brake system does the supply tank feed?
The primary and secondary reservoirs of the brake system.
Each is pressure protected by means of a one-way check valve.
How do you verify one-way check valve operation?
Drain the supply tank and watch for pressure loss in the other reservoirs.
How are one-way check valves used throughout an air-brake system?
To pressure-protect and isolate portions of the circuit.
What does two-way (double) check valves do?
Two-way check valves provide a means of providing the primary circuit with secondary circuit air and vice versa, in case of a circuit failure.
Is a T with two inlets and a single outlet.
It outputs the larger of the two-source pressures to the outlet ports and blocks the lower pressure source; the valve will shuttle in case of a change in the source pressure value.
What should the technician do before performing a pressure drop test on a heavy truck?
Ensure the engine is stopped and the system is at full pressure.
How do you perform a pressure drop test on a heavy truck?
Make and maintain a brake application.
Allow pressure to stabilize for one minute and then begin timing for two minutes while watching the dash gauge.
What is the pressure drop for single vehicles?
4 psi within two minutes while watching the dash gauge.
What is the pressure-drop for singles’ combination vehicles?
6 PSI within two minutes.
What federal legislation defines system air buildup times?
What does a common check performed by enforcement agencies require?
The supply circuit on a vehicle be capable of raising air system pressure from 85 to 100 psi in 2 seconds or less.
Failure to achieve this buildup time indicates:
a worn compressor
a defective compressor unloader assembly
supply circuit leakage,
or a defective governor.
What does FMVSS 121 require when the system pressure drops below 60 psi
FMVSS 121 requires that a driver receive a visible alert, in most cases this is accompanied by an audible alert, usually buzzer.
A low air pressure warning device is fitted to both the primary and secondary circuits..
The switch is electrically closed whenever the air pressure being monitored is below 60 psi. When the air pressure exceeds 60 psi, the switch opens.
How can you verify the operation of a low air pressure warning switch?
This can be done by pumping the service application valve until the system pressure drops to the trigger value.
How would you check for the proper functioning of the inversion (spring brake control) valve?
The air system should be fully charged (Min. 100 psi), and the spring brakes must be released.
Next, locate and drain the wet tank and then the primary axle service reservoir completely by means of the draincocks.
The primary gauge needle will drop to 0 psi, indicating a pressure loss for that system, which is normal.
The secondary gauge needle must remain at system pressure (min. 100 psi) to start the check of the inversion valve.
Depress the foot valve in a normal manner.
The spring brakes on the rear axle (s) will apply as the air is exhausted from the brake chambers.
Release the foot valve, and the spring brakes do not or only partially release, then the inversion valve has failed and should be replaced.
What does the tractor protection valve do?
The tractor protection valve protects the tractor air supply under a trailer breakaway condition or severe ait leakage.
Which two air lines connect a tractor brake system with a trailer air circuit?
The trailer supply line supplies the trailer with air for braking and any other pneumatic systems such as its suspension.
The trailer service line is the service brake signal line.
Both these lines are plumbed to the tractor protection valve.
When the trailer supply dash valve (emergency) red hexagonal is in its off position the tractor protection valve is also off.
What happens when the driver makes a service brake application?
No air will exit the tractor protection valve service signal line to the trailer.
What happens when the trailer air supply valve is open?
Air will open the tractor protection valve and supply the trailer pneumatic system with air.
Then when the driver makes a service brake application, the service brake signal air will be transmitted through the tractor protection valve to apply its service brakes.
The trailer air supply controls the trailer park brakes. So what happens whenever air supply is interrupted?
When the trailer is being parked or unintentionally in a breakaway, the spring brakes will apply regardless of how much pressure is in the trailer air reservoirs
From what is the tractor protection valve designed to isolate?
The tractor protection valve is designed to isolate the tractor air system from that of the trailer at a predetermined value that ranges from 20 to 45 psi, depending on the system.
These pressures, the spring brakes on both the tractor and trailer would be partially applied.
Whatis a common check performed by law enforcement agencies in regards to the supply circuit on a vehicle?
Law enforcement agencies requires that the supply circuit on a vehicle be capable of raising air system pressure from 85 to 100 psi in 25 seconds or less.
What could be the problem if the supply circuit on a vehicle fails to raise air system pressure from 85 to 100 psi in 25 seconds or less?
Failure to achieve the build-up time indicates a worn compressor, defective compressor unloader assembly, supply circuit leakage, or a defective governor.
What is the trailer control valve used for?
To actuate the trailer service brakes on a trailer independently of the tractor service brakes. This valve is also known by the terms: trolley valve, broker brake, and spike.
What is the source of air supplied to the trailer control valve?
It is usually the secondary circuit. Brake application pressure air is modulated proportionally with control valve travel.
What are two-way check valves used for and where are they located?
Two-way check valves are used in the application circuit of the trailer brakes. They are located downstream from both the trailer control valve and the treadle valve.
In a typical system, what sourced air does the trailer application brake valve (trolley or spike) use, and what sourced air does the treadle valve trailer service signal use, to activate the trailer service brakes?
In a typical system, the trailer application bake valve uses secondary circuit sourced air and the treadle valve trailer service signal uses primary circuit sourced air to activate the trailer service brakes.
If a driver was using the spike to bring the vehicle to a stop and during this process, an emergency occurred that required a panic application of the treadle valve, , what will happen?
The two-way check valve would permit the source air to change from secondary circuit air from the trailer control valve to the higher source pressure value from the treadle valve.
What would you use to verify the trailer control valve performance?
An accurate air pressure gauge. The trailer is supplied with air by means of air hoses.
Service and trailer supply air are sent to the tractor protection valve.
How would you perform the antilock brake system (ABS) operational system self-test?
All current ABSs are electronically managed.
a typical ABS adapts a standard air or hydraulic system for electronic management. All current systems are designed to default to non-managed operation in the event of an electrical or electronic malfunction. (New trucks are equipped with ABS brakes and are mandatory.
What does ab ABS consist of?
An ABS consists of a means of monitoring the speed of each wheel, an electronic control unit (ECU) or ABS module to manage the system, and a modulator assemble to affect the outcomes of the ECU logic processing by modulating the pressures to each wheel.
ABS brakes are especially effective at managing split-coefficient braking conditions. When does split coefficient braking occur?
Split coefficient occurs when road surface conditions (icy, wet, gravel, dry, etc) differ from one wheel to another.
What are all ABSs equipped with and how can these be accessed?
Self-diagnostics. These can be accessed by flash or blink codes using the dash warning light or a scan tool (digital diagnostic reader) connected either at the ATA connector in the cab or directly to the ABS module.
What does the self-test consist of when an electronic ABS is powered up.
The self-test consists of testing the signal range in the wheel speed sensors and cycling the solenoids in the system actuators.
How would you access the ABS self-diagnostics?
Access the ABS self-diagnostics by selecting the correct message identifier (MID).
Identify the failure mode indicator (FMI) that has triggered the fault code and repair the failure using the OEM service instructions.
How would you inspect coupling air lines, holders, and gladhands?
- Visually check all hoses and lines in the brake system.
- Service and trailer supply air is plumbed from the tractor protection valve.
- This air is transferred to the trailer by air hoses with couplers known as glad-hands. Gladhands enable coupling between the tractor air supply and the trailer pneumatic circuit.
Gladhand seals are usually manufactured out of rubber.
They are retained in an annular groove in the gladhand and are easily replaced when they fail.
What are some of the deficiencies that should be noted during an annual inspection of the brake chambers, and air lines?
- Absence of braking action on any axle, required to have brakes, upon application of the service brakes.
- Missing or broken mechanical components including shoes, lining pads, spring, anchor pins, spiders, cam rollers, pushrods, and air chambers mounting bolts.
- Loose brake components including air chambers, spiders, and camshaft support brackets.
- Audible air leak at brake chamber (ruptured diaphragm, loose chamber clamp, etc.
- Hose with any damage extending through the outer reinforcement ply. Rubber impregnated fabric cover is not a reinforcement ply. Thermoplastic nylon may have braid reinforcement or color difference between cover and inner tube, Exposure of second is cause for rejection.
- Bulge or swelling when air pressure is applied.
- Any audible leaks from hose or tubing.
- Two hoses improperly joined (such as a splice made by sliding the hose ends over a piece of tubing and clamping the hose to the tube.)
- Air hose cracked broken or crimped.
- Tubing cracked, damaged by heat, broken or crimped.
What are the conditions of brake linings that cause them to need to be replaced?
- The lining or pad is not firmly attached to the shoe.
- Saturated with oil, grease or brake fluid.
- Non-steering axles: Lining with a thickness less than 1/4 inch at the shoe center for air drum, brakes, 1/16 inch or less at the shoe center for hydraulic and electric drum brakes, and less than 1/8 inches for air disc brakes.
- Steering axles: Lining with a thickness less than 1/4 inch at the shoe center for drum brakes, less than 1/8 inch for air disc brakes, and 1/16 inch or less for hydraulic disc and electric brakes.
Unequal lining wear should be noted and the cause investigated.
How are truck brake shoes mounted and actuated?
Truck brake shoes are in most cases fixed-anchor assemblies mounted to the axle spider and actuated by an S-camshaft.
Brake shoes for a heavy-duty truck can have their friction facings or linings mounted to the shoe by bonding, riveting, or by screw fasteners.
How are current application shoes remanufactured and replaced?
Current application shoes are remanufactured and replaced as an assembly, that is, with the new friction facing already installed.
What should you check for when reusing shoes?
Check for ar deformities usually caused by prolonged operation without brake adjustment.
What should you do when reconditioning shoes with bolted linings?
When reconditioning shoes with bolted linings, if the original fasteners are reused, the lock washers should at least be replaced.
Riveted linings should be rivet in the manufacturer’s recommended sequence.
How is the friction rating of linings coded?
By letter codes.
Combination lining sets of shoes are occasionally used; these use different friction ratings on the primary and secondary shoes.
When combination friction lining sets are used, care should be taken to install the lining blocks in the correct locations on the brake shoes.
What is a good PMI practice for a tandem drive axle truck?
Replace all of the brake linings on a PMI schedule.
When the linings on a single wheel are damaged, as in the case of wet seal axle lube failure, what should you do?
The linings of both wheel assemblies on the axle should be replaced to maintain brake balance.
How should you change the friction pads in air disc brake assemblies?
The friction pads in air disc brake assemblies should be changed in paired sets, that is, both wheels on an axle.
Vented disc brakes may have thicker inner pads than outer pads while solid disc brakes usually have equal thickness inner and outer pads.
Heat transfers more uniformly in the solid disc assemblies than vented ones, requiring thicker inboard pads.
What are the different factors that brake drum condition can vary?
Including, but not limited to, scoring, bell-mouth condition, concave-like, convex-like, threaded appearance, and hard spots.
What does a convex condition indicate?
Indicates a drum diameter greater at the friction surface edges as compared to the center of the friction surface.
What does a concave condition indicate?
Indicates a drum diameter greater at the center of the friction surfaces as compared to the edges.
When does a bell-mouthed condition occur?
This occurs when the drum diameter is greater at the inboard edge of the drum (next to the backing plate) compared to the outboard edge.
What is an out-of-round condition?
This is a variation in the drum diameter at measurements taken 180 degrees apart.
How do you check for lateral run out?
A dial indicator is usually mounted on the hub and the face of the rotor is indicated. Total indicated run out should generally not be more than 0.005 inch (0.13 mm)
If brake drums are to be reused, what should should you inspect for?
Reused brake drums should be inspected for wear, heat discoloration, glazing, threading, concaving, convexing, bell mouthing, and heat checking.
When is it a good practice to machine new drums?
Before installation due to warpage caused by incorrect storage practices after manufacture.
What is the maximum legal service limit for 14-inch-diameter brake drums?
What is the machining limit?
What is the discard limit?
The maximum legal service limit for 14-inch-diameter brake drums is 14.120 inches.
The machining limit is 14.060 inches.
The discard limit is 14.090 inches.
What must brake rotors be visually inspected for?
For heat checking, scoring, and cracks.
How do you measure rotors?
Measure rotors for thickness with a micrometer.
Use a dial indicator to check the rotor run out and parallelism.
In highway applications, rotors tend to outlast drums and are often reused after a brake job.
They must be turned within legal service specifications using a heavy-duty rotor lathe.
What are slack adjusters critical for?
Maintaining the required free play and adjustment angle.
Current slack adjusters are required to be automatically adjusting, but their operation must be verified routinely.
Since manually adjusting automatic slack adjusters is dangerous and should not be done, when can it be done?
During installation or in an emergency to move the vehicle to a repair facility, because manual adjustment of this brake component:
1, fails to address the true reasons why the brakes are not maintaining adjustment, giving the operator a false sense of security about the effectiveness of the brakes, which are likely to go out of adjustment again soon. and
2. causes abnormal wear to the internal adjusting mechanism for most automatic slack adjusters, which may lead to failure of this brake component.
What do slack adjusters connect?
Slack adjusters connect the brake chambers with the foundation assemblies on each wheel. They are connected to the pushrod of the brake chamber by means of a clevis yoke (threaded to the pin. purhrod_andpinn
How are slack adjusters mounted?
Slack adjusters are spline mounted to the S-cams and positioned by shims and an external snap ring.
What does the slack adjuster convert the linear force of the rod into?
The slack adjuster converts the linear force of the brake chambers rod into rotary force or torque and multiplies it.
What does the distance between the slack adjuster S-cam axis and the clevis pin axis define?
The leverage factor; The greater this distance, the greater the leverage.
What is the objective of brake adjustments, whether manual or automatic?
Is to maintain a specified drum to lining clearance and a specified amount of Freeplay.
What is Freeplay?
Freeplay is the amount of slack adjuster stroke that occurs before the linings contact the drum.
What lubricates slack adjusters?
Grease or automatic lubing systems lubricate slack adjusters.
How are the seals in slack adjusters always installed?
The seals in slack adjusters are always installed with the lip angle facing outward.
When grease is pumped into the slack adjuster, grease will easily exit the seal lip when the internal lubrication circuit has been charged.
What does failure to adjust the slack adjuster clevis with the proper template cause?
Improper self-adjusting action in the slack adjuster.
If necessary, adjust the clevis on the end of the pushrod until the outer hole in the clevis is completely visible through the template opening.
What should you do when performing foundation brake servicing?
All of the hardware mounted on the axle spider should be inspected and replaced if required.
There should be no radial play of the S-camshaft
Rollers and cams should be inspected for flat spots, and it is good practice to replace any spring-steel components such as retraction springs and snap rings.
Why should you install S-cam bushing seals?
So that the spider outboard bushing sealing lip faces inward toward the bushing and the inboard bushing sealing lip faces outward from the bushing.
This allows excess grease to exit from the inboard side of the bushing and prevent it fro being pumped into the foundation assembly.
Why should S-cam profiles never be lubricated?
S-cam profiles depend on friction and should never be lubricated.
What should be the procedure for lubrication if the slack adjuster passes inspection?
Apply a thin film of chassis grease to the slack adjuster splines.
After reassembly, pressure-lubricate the slack adjuster according to the manufacturer’s instructions.
Pressure-lubricate the in-board S-cam bushing until grease flows out of the slack adjuster end of the tube.
Grease should not flow out of the out-board end of the tube.
If it does the seal is defective and it must be replaced.
Do not replace the existing grease fitting(s) with the pressure relief type.
Only standard non-vented fittings are to be used with spring-loaded lip seals.
When checking hydraulic brake fluid in a vehicle what should be checked?
When checking hydraulic brake fluid, not only should the level be checked, but any leaks from the master cylinder reservoir, or brake line fittings as well.
Why is the appearance of the fluid an important part of the inspection?
A cloudy or opaque look to the fluid will indicate some sort of contamination (dirt rust, etc.), especially if severe. any fluid that is cloudy or darkened should be flushed and refilled with fresh fluid.
What would you look for when visually checking around the wheel cylinders?
Check for an accumulation of contamination, which generally indicates leakage.
What is a thorough inspection of brake lines considered an important part of?
The hydraulic brake system.
When checking the parking brake controls, what should you do?
Test actuator Freeplay and the parking brakes’ ability to effectively stop the vehicle.
How should the brake pedal feel when checking its operation?
The brake pedal should feel firm with minimal free travel at the top of the stroke.
If it feels spongy it is usually an indication of air in the hydraulic system.
What are trucks with hydraulic brakes manufactured after 1976 equipped with?
A tandem master cylinder, and dual circuit brakes.
In a dual circuit brake system, when one circuit fails, the remaining circuit will be unaffected.
When there is a failure in one circuit, the brake pedal will have excessive free travel.
A dash warning light should also be illuminated.
What may be occurring if the pedal slowly drifts to the floor?
There may be an internal leak in the master cylinder or an external fluid leak.
When the pedal is depressed, what should happen?
The brakes should apply promptly without excessive effort.
What should be checked if excessive pedal effort is required to stop the truck?
The power brake booster.
How should all of the wheels respond when the brakes are applied?
All of the wheels should brake evenly and the truck should not pull to either side.
If there is a pull, or one wheel locks up, or skids, the problem should be located and repaired before the truck is dispatched.
When checking the wheel cylinders for an accumulation of contamination, which generally indicates leakage, what should you look for?
Look for evidence of leaks on the inboard side of the tire and wheel.
When the drums are removed as part of a PMI, how should you check the wheel cylinders?
Peel back the rubber boot and check for fluid. Leaking calipers may be examined with the wheel removed.
What can leakage from wheel cylinders result in?
Brake failure. Leaking wheel cylinders must be reconditioned or replaced.
Most trucks are equipped with power brakes to provide shorter stopping distances with less effort. What provides power brake assist?
A vacuum booster, a pneumatic booster, or a hydraulic booster. Hydraulic boosters are more effective and more common.
When road testing the truck, it should stop without excess pedal effort.
There should also be reserve capacity, providing for at least one power-assisted brake application after the engine is off.
What does the hydro-max power brake booster accomplish?
It engages an electro-hydraulic pump when there is an interruption of fluid flow from the engine-driven hydraulic pump.
The hydraulic power source may e the power steering pump or may be a dedicated hydraulic pump.
Why do trucks with diesel engines and vacuum boosters need an engine-driven vacuum pump?
Because a diesel engine has pressure, not vacuum in the intake manifold.
What does air over-hydraulic systems use for power assist?
Air pressure and the air in the tanks provide reserve capacity.
What are the control valves required for braking balance with disc/drum combinations?
The metering valve delays the application of front disc brakes until the rear drum brakes are ready to apply.
If only the front brakes lock up or skid on slick surfaces when the brakes are lightly applied, the metering valve should be checked.
How can you check the metering valve?
By installing pressure gauges on the front and rear brake circuits.
There should be no pressure at the front until the pressure at the rear reaches about 30-50 psi.
Why is a proportioning valve needed?
To reduce rear-wheel lock-up during moderate to heavy brake applications.
What do load-sensing proportioning valves do?
Allow more pressure to the rear wheels as the frame is lowered by heavier loads. The rear wheel should not lock up before the front.
There should be balanced braking at each of the truck’s wheels.
What does a pressure differential valve do?
Turns on a dash warning light when a loss of pressure occurs in one half of a split hydraulic brake system.
What do combination valves do?
Combine the metering, proportioning, and pressure differential valves into one assembly, which is replaced as a unit.
How do you adjust hydraulic drum brakes?
- Remove the backing plugs for measuring the shoe-to-drum clearance. Be sure that, if used, the parking brake chambers are caged.
- Apply and release the brake pedal several times yo center the brake shoes.
- Insert a feeler gauge through the backing plate opening and measure the clearance between the brake shoe and the drum. Typically this clearance should be 0.040 to 0.100 inch (1.01 to 2.5 mm)
- If the shoe clearance is not correct, turn the manual adjusting heel (star wheel) on the wheel cylinder to obtain the correct clearance.
Why should trucks equipped with hydraulic-assist power brakes (usually hydraulic pressure is supplied from the power steering pump) require back-up to supplement power braking?
It would be needed In the event the engine stops while the truck is moving.
This is accomplished by a flow switch, which senses the loss of hydraulic pressure from the power steering pump and actuates a 12-volt electro-hydraulic pump to provide emergency power assist.
How can you test the hydraulic assist back-up system and warning devices?
On some systems, this is tested by making several brake applications with the key on and the engine off
When you have made enough pedal applications to exhau=st the pressure in the accumulator, you should get a warning light and busser, and hear the 12-volt electro-hydraulic pump engage.
Other systems are controlled by a solid-state alarm/brake booster module and can verify when they are operating normally by observing both a primary and a secondary brake light and a tone alarm.