Hydraulic Brakes Diagnosis and Repair Flashcards
(148 cards)
1
Q
Where does the potential energy of a hydraulic brake system come from?
A
The potential energy of a hydraulic brake system comes from the mechanical force created by the action of a driver’s foot on a brake pedal, usually assisted proportionally by pedal geometry leverage and a power-assist system.
2
Q
What can be assumed in any hydraulic circuit?
A
The hydraulic medium is not compressible.
3
Q
What happens if force is mechanically applied to a liquid in a closed system?
A
It will be transmitted equally by the liquid to all parts of the system.
4
Q
What happens if forced is applied by a master cylinder?
A
It will transmitted equally throughout the hydraulic system.
Valves are used to modify that pressure in some locations.
5
Q
What does the hydraulic circuit of a hydraulic brake system consist of?
A
A master cylinder, proportioning valves, metering valves, pressure differential valve, and wheel cylinders.
6
Q
All truck hydraulic brake systems have dual circuits. What does this mean?
A
In the event of a failure in one of the circuits, the other will back it up.
As in truck air brake systems, the circuit are defined as the primary and secondary circuits.
7
Q
What can cause poor stopping?
A
Poor s
8
Q
What can cause noise?
A
Noise can be caused by loose or incorrectly installed components.
9
Q
What can be the cause of pulling, grabbing, and dragging?
A
sticking valves and collapsed hoses.
10
Q
What can cause hard brake pedal concerns?
A
Hard brake pedal concerns can be caused by a faulty booster.
11
Q
What can cause soft brake pedal concerns?
A
can be caused by air in the system, a leak, or a bypassing master cylinder.
12
Q
What would you use to test and determine the cause of pedal concerns?
A
Use a set of hydraulic gauges to determine the cause of the concern and the repair needed.
13
Q
What may be used to test pressure values within the hydraulic circuit?
A
pressure gauges.
14
Q
How can the hydraulic system circuit be pressurized?
A
The hydraulic system circuit can be pressurized simply by starting the vehicle engine and applying the brakes by foot pressure.
15
Q
How can you verify external leaks?
A
External leaks may be verified by cleaning the externally visible portions of the circuit and applying the brakes.
16
Q
How can you verify internal leaks?
A
Internal leaks are more difficult to locate.
Internal leakage within a master cylinder can be verified by using gauges plumbed to each portion of the hydraulic circuit.
17
Q
What would you use to retrieve and diagnose fault codes?
A
A handheld scanner or a PC with the appropriate software.
The tool is connected to the ATA data link, which is usually located in the cab typically to the left of the steering column.
Fault codes can be diagnosed using the manufacturer’s troubleshooting trees.
18
Q
What do most truck hydraulic brake system, use the brake pedal assembly to do?
A
Add leverage to the mechanical force provided by the driver’s foot pressure.
19
Q
What happens if you have insufficient pushrod or a pushrod that is adjusted too tight?
A
Can force the master cylinder pistons forward slightly, blocking the compensating ports.
This prevent fluid flow between the master cylinder bore and the reservoir, which can cause brake pressure buildup from heat expansion of the fluid resulting in brake drag.
Freeplay should be adjusted to the manufacturer’s specification.
20
Q
How are the primary and secondary circuits in the brake system actuated?
A
The master cylinder converts the mechanical force applied to it by driver foot pressure and the brake booster system into hydraulic pressure.
21
Q
What do the primary and secondary circuits of the brake system consist of?
A
It usually consists of integral reservoirs (one for each circuit), cylinder housing, compensating ports, return springs, and primary and secondary pistons.
22
Q
How is the primary piston actuated?
A
The primary piston is actuated mechanically.
Pressure developed in the primary portion of the master cylinder charges the primary circuit (this can actuate either the front or rear ) and the secondary piston.
23
Q
When is the secondary piston actuated?
A
Is actuated hydraulically by whatever pressure value is developed in the primary portion of the cylinder to charge the secondary circuit.
24
Q
What happens when the mechanical force applied to the primary piston is relieved?
A
Return springs acting on both the primary and secondary pistons return them to their original positions, permitting the fluid applied to each circuit to return to the reservoirs.
25
How are both sections of the master cylinder aspirated?
They are aspirated with brake fluid by fill and compensating ports.
26
How is each piston sealed in its bore?
Each piston is sealed in its bore by rubber seals.
| Primary and secondary circuit fluid do not come into contact with each other under normal operation.
27
What happens when a failure occurs in either circuit?
The pressure differential switch will illuminate the red lamp in the dash the first time the brakes are applied following the failure.
28
What happens when a failure occurs in the primary circuit?
The primary piston will be forced through its travel without generating any fluid pressure until it contacts the secondary piston and mechanically actuates the secondary circuit
29
What happens if a failure occurs in the secondary piston
The primary circuit will function normally, but the actuation of the secondary piston will result in no pressure delivered to the secondary circuit.
30
What should you do to the vehicle in either case of a primary or secondary failure?
The vehicle should be brought to an immediate standstill and not operated until a repair has been performed.
31
What should you use when testing a master cylinder in a hydraulic braking system?
When testing a master cylinder in a hydraulic braking system, a liquid-filled hydraulic test gauge should be used.
32
What can cause sludge and particulate that can plug fill and compensating ports, resulting in slow application times, slow release time, and brake failure?
Deteriorated fluid, deteriorated seals, or a mixture of incompatible fluids.
33
Master cylinders are generally serviced by replacement for time and convenience reasons but they can also be ________________.
Reconditioned.
34
What will swell the seals and leave behind corrosive residues?
Use of any petroleum-based lubricants or solvents.
35
When replacing a master cylinder, what should be checked?
The brake pedal pushrod length should be checked when replacing a master cylinder.
36
What will a pushrod adjusted too long cause?
A pushrod adjusted can cause the brakes to drag.
37
What will a pushrod adjusted too short cause?
A pushrod adjusted too short can prevent sufficient travel of the master cylinder piston.
38
When inspecting and replacing brake lines, flexible hoses, and fittings, what should steel tubing be checked for?
Steel tubing should be checked for wear, dents, kinks, and corrosion.
39
What is the benefit of preflared and preformed tubing?
Preflared and preformed tubing helps reduce custom cuts, bending, and flaring of new tubing.
40
What are two types of flaring styles and seats?
International Organization for Standardization (ISO) and double flare.
ISO uses an outward flare, while a double flare creates a double wall at the nipple seat for greater strength.
41
What should you clean brake tubing with and why?
Only isopropyl alcohol should be used because of its ability to evaporate rapidly and residue-free drying.
Residues remain when using other cleaning agents such as soap and water or mineral spirits.
42
What should use to bend tubes?
A tube bender.
43
What will kinked lines cause?
Kinked lines create flow restrictions that can affect brake performance.
44
When should you replace flexible hose?
Flexible hose should be replaced when visibly cracked, bulges under pressure, or is leaking.
45
What should you do if brake fluid is contaminated with a petroleum-based liquid?
The flexible hoses and all other rubber components of the brake system must be replaced.
46
How should brake lines be routed?
Brake lines should be routed away from all moving components and care should be taken to ensure that nothing will contact the lines during suspension jounce and rebound or during steering.
47
why is a metering valve used on vehicles equipped with front disc and rear drum brakes?
It is required to achieve brake timing balance during light brake applications by withholding the delivery of application pressure to the front disc brakes until pressure exceeds a predetermined value in the circuit responsible for actuating the rear brakes.
48
Why is a lag or delayed required through withholding the delivery of application pressure to the front disc brakes?
The lag or delay is required so that hydraulic pressure builds sufficiently in the rear brake hydraulic circuit to overcome the tension of the rear brake shoe return springs and the free travel the shoes.
The objective is to enable simultaneous application of both front and rear brakes.
For this reason, the metering valve is sometimes known as a hold-off valve.
49
What should you do when using a pressure bleeder to bleed any system equipped with a metering valve?
The manufacturer's instructions as to how to open the valve must be observed.
50
What should you do when manually bleeding a brake system?
When manually bleeding a brake system, application of the brake pedal develops sufficient pressure to overcome the metering valve opening pressure.
51
What is a portioning valve used on and what is its functioning?
A proportioning valve is used on systems combining front disc and rear drums.
the proportioning valve is installed in the circuit supplying the rear brakes.
Its function is to reduce the application pressure to the rear wheel cylinders and prevent rear wheel lockup.
52
When should the proportioning valve be verified?
The proportioning valve should be verified at each brake inspection or if reported for a rear wheel lockup condition.
To check valve operation, hydraulic gauges should be installed ahead and behind the valve.
53
Why is a load-or-height-sensing valve used on some systems?
To sense vehicle load transfer effect under braking.
54
How does a load or height sensing valve work?
The valve proportions front and rear braking, correlating it to weight transfer during braking..
55
Where is load or height sensing valve located?
The valve is located on the vehicle frame cross member and is activated through a linkage system connected to the rear axle housing.
56
How are the metering (hold-off) load sensing/proportioning, proportioning, and combination valves checked?
These valves are checked by installing pressure gauges at each wheel end and comparing gauge pressure when applying and releasing the brakes.
A faulty valve can prevent pressure buildup at one or more wheels, cause pressure to build too quickly, or trap the pressure and cause the brake to drag.
57
What does the pressure differential valve consist of?
The pressure differential valve is also known as a brake light warning valve.
It consists of a cylinder through which primary and secondary hydraulic pressure acts on either side of a spool.
58
What happens when the pressure in both the primary and secondary circuits is equal?
The spool floats in a neutral position.
59
What happens when a pressure imbalance occur in either the primary secondary circuit?
The spool will shuttle to one side of the cylinder, and in doing so grounds an electrical signal, illuminating a dash warning lamp.
60
How can the electrical circuit in a valve be checked?
By removing the wire from the valve and grounding it.
61
When checking the electrical circuit, what s happening if the warning light was off and then came on?
Then the circuit is working correctly.
62
If the concern is the light is on continually, what should you do?
Disconnect the wire from the sensor.
63
What should you do if the concern is the light is on continuously?
Disconnect the wire from the sensor.
64
What is going on if the light goes out or does not go out?
If the light goes out, the sensor or valve is the cause for the concern.
If the light does not go out, a ground in the circuit from the switch to the bulb could be the cause.
65
What does a wheel cylinder actuate?
A wheel cylinder actuates a hydraulic brake system.
It is supplied with hydraulic brake fluid supplied by the master cylinder and converts hydraulic pressure to mechanical force at the foundation brake assembly.
66
What are wheel cylinders usually constructed of?
Wheel cylinders are usually constructed of cast iron for higher durability and lower manufacturing costs.
67
Most wheel cylinders are double acting. What do they house?
Wheel cylinders house two pistons within a cylinder bore.
| The pistons are sealed in the cylinder by rubber seals.
68
What happens when pistons are subjected to hydraulic pressure?
They are forced outward to actuate brake shoes, forcing them into the drum.
69
What mechanisms are often integral parts in the wheel cylinder assembly?
Manual and automatic adjusting mechanisms are often integral parts in the wheel cylinder assembly.
70
Why are wheel cylinders generally replaced?
the cylinder bores become scored, the result is fluid leakage from the cylinder past the seals.
Inspect for this condition by loosening the dust cover on the wheel cylinder and looking for fluid behind it.
71
What can make seals fail?
Seals may fail if exposed to chemical contaminants.
72
What happens if dampness and moisture cause a buildup of rust or corrosion in the bore?
Dampness and moisture can cause a buildup of rust or corrosion in the bore, resulting in seizure of the wheel cylinder pistons and no brake actuation.
73
What must you do when replacing the wheel cylinder?
When replacing the wheel cylinder, air must be bled from the cylinder and brake lines after installation.
Each wheel cylinder is equipped with a bleed port.
Loosen this bleed port to purge air from the hydraulic circuit in the bleeding process.
74
Fixed and sliding or floating calipers are used in hydraulic brake-systems.
What happens in a fixed caliper?
In a fixed-type caliper, the rotor sits in between two or four pistons, and the clamping action of opposed pistons acting on the rotor is responsible for retarding the rotor.
75
What happens in a floating or sliding caliper?
In a floating or sliding caliper, one or more pistons sit on one side of the rotor, and the caliper housing is designed to slide when the piston is subjected to hydraulic pressure.
This action affects the clamping action of the rotor.
76
What are the friction surfaces that contact the rotor?
The friction surfaces that contact the rotor are brake pads
77
Are disc brakes self-energizing?
Disc brakes are not self-energizing, and in general greater hydraulic pressure is required to apply them?
They operate at higher mechanical efficiencies than drum brakes.
78
What should calipers be inspected for?
Calipers should be inspected for brake fluid leaks, binding, or looseness on the slides.
Disc brake calipers are usually replaced instead of being rebuilt.
79
What should you do and look for when replacing a caliper?
When replacing a caliper, care should be taken to install the correct caliper on the correct side.
Installing the wrong caliper on the wrong side can result in the bleeder screw being at the bottom instead of at the top.
This makes the brake system practically impossible to bleed.
80
Brake fluid should be tested on a regular interval, some manufacturers recommend testing annually. Hw would you perform testing?
Test can be performed with test strips.
| it is a good practice to replace the system brake fluid at each major brake overhaul.
81
What are the characteristics of approved heavy-duty brake fluid?
The approved heavy-duty brake fluid should retain the proper consistency at all operating temperatures.
It will not damage rubber cups and helps to protect the metal parts of the brake system against failure.
82
Why should water, mineral spirits, and gasoline never be used to flush a hydraulic braking system?
Because of incompatibility with other materials and corrosive effects.
83
What is used to flush hydraulic braking systems?
Alcohol or compatible brake fluid is always used to flush hydraulic braking systems, and these fluids should not be reused after the flushing is complete.
84
Why should a container storing brake fluid always be tightly sealed when not in used?
To prevent moisture from being absorbed into it.
85
What does mineral oil, alcohol, antifreeze, cleaning solvents, and water do to most brake fluids?
Contaminates brake fluids even in very small quantities.
86
What is the shelf life of brake fluid?
Shelf life of brake fluid is one year.
| This shelf life diminishes if the container is not completely full.
87
What happens if you mix different brake fluids?
Mixing of different brake fluids can cause coagulation and result in hydraulic brake failure. It is important to ensure that fluids are compatible when topping up system reservoirs.
88
How do you bleed brake systems?
Brake systems may be bled using a bleeder ball or manually.
The first method (using a bleeder ball), is the preferred method as one person can perform the operation.
In all cases system air is purged from valves and wheel cylinders in a sequence defined by the manufacturer.
Typically this sequence starts with the wheel farthest from the master cylinder.
89
What is servo action?
Servo action occurs when the action of one shoe is guided by the movement of the other, permitting both shoes to act as a single unit.
90
When does self-energizing occurs?
Self-energizing occurs when the shoes are driven into the drum and rotate fractionally as a pair with the drum, before grabbing.
91
What is non-servo action?
Non-servo action is also a term generally used for certain drum/shoe-type brakes.
Some original equipment manufacturers (OEMs) refer to these brakes as leading trailing shoe brakes.
92
How does each shoe on non-servo brakes work?
Each shoe on non-servo brakes works independently of each other to stop the vehicle. They are separately anchored.
When actuated by the wheel cylinder, the shoe pivots on the anchor and is faced against the drum.
93
What is the difference between disc brakes and drum brakes?
Disc brakes are non-energized, and they require more force to achieve the same braking effort as self-energized drum brakes.
94
How do disc brakes obtain the extra force to achieve braking effort?
To obtain this extra force, most hydraulic truck calipers contain two large or four medium sized apply pistons.
They do have superior mechanical efficiency and are often used on the front axle brakes of straight trucks.
95
What causes grabbing and pulling in the foundation of the brake assembly?
Grabbing and pulling can occur in the foundation brake assembly caused by broken hardware components, binding disc brake calipers, loose wheel bearings, contaminated linings, drum failures, malfunctions in the adjusting mechanism, and parking brake related problems.
96
What is the primary cause of a pulsating pedal condition?
A warped disc brake rotors. Rotors can warp due to overheating or machining rotors too thin at overhaul.
97
What causes premature wear on one wheel end?
Can be caused by failure of the brakes to apply at one wheel end thus overworking the other wheel ends.
98
What are other causes of premature wear on one wheel end?
Other causes of premature wear are overloaded vehicles, operator error, incorrect brake linings, and dragging brakes.
99
What causes noise?
Can be caused by missing clips or retainers, incorrectly position pads, or excessively worn pads, shoes, rotors, or drums.
100
Brake drums may be reused if they are within the manufacturer's specifications. What are the critical specifications?
The critical specifications are the maximum wear limit, machine limit, and maximum permissible diameter.
101
How are drums measured?
Drums are measured with a drum gauge and should be checked for out of round, bell-mouthing, convexing, con-caving, and taper.
102
After measuring, what should drums be inspected for?
Drums should be inspected after measuring for heat checks and cracks before they are machined.
Worn drums are normally replaced instead of being resurfaced.
103
When may disc brake rotors be reused?
Disc brake rotors may be reused if they are within the manufacturer's specifications.
104
How should disc brakes be measured?
They should be measured for thickness with a micrometer and checked for parallelism and run-out with a dial indicator.
If within machine limits, the rotor may be turned on a rotor lathe.
105
What is typically done when a brake job is performed?
Typically, when a brake job is performed, the brake shoes, return springs, and fastening hardware are replaced.
106
What should be observed when replacing brake shoes?
The friction edge codes should be observed when replacing brake shoes.
Ensure that primary (leading) and secondary shoes are installed in their correct locations.
107
What are the first components to wear out in duo-servo brake systems?
Brake shoes are the first components to wear out in duo-servo brake systems.
108
What should backing plates be inspected for?
Backing plates should be inspected for grooves, loose pins, and worn holes.
Adjuster mechanisms should be cleaned and lubricated with the recommended lubricant.
109
How will you know that the wear limit is exceeded on many disc brakes.
Many disc brake assemblies have wear indicators that produce a squealing noise when the wear limit is exceeded.
110
Was does servicing disc brakes pads often involves ?
removing the caliper assembly, ensuring that float pins are not seized, and installing a new pair of brake pads.
111
What should you do whenever the brake pads are replaced?
Whenever the brake pads are replaced, the brake rotors should be both measured and visually inspected.
To ensure acceptable brake performance replacement pads should be of the same construction as the original pads.
112
What should you do when installing new brake pads?
New brake pads should be burnished after installation.
The old pads should be inspected for wear patterns.
Pads that are worn at an angle can indicate a bent caliper mount.
113
When are driveline parking brakes used?
Driveline parking brakes are used in some hydraulic truck brake systems, especially in air over-hydraulic brake systems.
114
Describe driveline braking systems.
Is a band and rotor or drum and shoe assembly mounted at the rear of the transmission or input to the differential.
When engaged, the rear wheels are locked stationary by means of the driveline.
115
What inspection does the band and rotor style require?
Inspection inspection of the rotor surface and the lining thickness.
The rotor's friction surface should be free of rust, scoring from rivets or signs of overheating from dragging linings.
116
What inspection does the drum brake style require?
The drum brake style requires all of the same checks as a regular drum brake assembly.
The drum brake should be inspected for out of round (unequal diameter), concave friction surface, convex friction surface and bell mouthing, which has the diameter at the edge of the drum larger than the diameter at the center or flange side.
117
What should the brake shoes be checked for?
The brake shoes should be checked for thickness, glazing, and lining separation.
118
Some truck hydraulic brake systems are equipped with a hydraulic parking brake system. How does a hydraulic parking brake system work?
These use a dedicated hydraulic circuit and have a parking brake cylinder located in the foundation assembly on the opposite side of the shoe to the wheel cylinder.
The parking brake is actuated by a cab-located control.
119
How does the hydraulic parking brake systems warn if there is insufficient pressure to cage the parking brake springs?
These systems will have low pressure warning devices to inform the driver if there is insufficient pressure to cage the parking brake springs.
120
In a truck hydraulic brake systems, why is the use of vacuum or hydraulically assisted brake booster required?
To reduce the foot effort that must be applied to the master cylinder to actuate the brakes.
121
Where are vacuum-assisted boosters usually not found?
on today's medium duty trucks using hydraulic brakes.
122
How is a hydraulic booster mechanism powered?
A hydraulic booster mechanism is powered by either the truck's power steering pump or by a dedicated pump.
123
What is a hydraulic booster comprised of?
The unit comprises an open center valve, reaction feedback mechanism, a large diameter boost or power piston, a reserve electric-powered pump, and integral flow control switch, and a power steering gear, operating in series.
124
How would you check the pump and valves?
The pump and valves can be checked for proper operation using a power steering pressure/flow meter.
Pressure will be low without brake pedal operation.
125
What happens when the operator applies pressure to the brake pedal?
A restriction is created in the booster assembly, and the, and the pressure from the pump will rise.
126
How does power brake boosters operate?
Power brake boosters operate by assisting pedal effort with hydraulic pressure in proportion to pedal travel.
127
What does malfunctioning hydro-boost units in trucks usually require?
Greatly increased brake pedal effort. The brake system cannot be effectively operated in this condition.
128
How would a technician diagnose a poor stopping concern?
The technician first needs to determine which style of pump assembly is used.
Some systems will use the power steering pump, while others will use a separate pump that is just for the brakes.
129
If the power steering pump is used to create pressure for the brake assist system, what is a quick method to help isolate the cause of poor power assist?
With truck running at idle, turn the steering wheel. If the power steering system is operating normally, the pump is not the cause of the poor power assist concern.
If the power steering system is sluggish, then the technician should focus on the pump and lines as the source of the poor stopping concern.
In a system that uses a stand-alone pump, the pump will need to be flow-and pressure checked using a hydraulic flow/pressure gauge.
130
How are electric back-up pumps typically energized?
Through a normally closed flow switch located in the booster.
If the flow switch is held open by brake assist fluid flow, then the electric pump is prevented from operating.
If the brake assist flow is interrupted, as in the case of a ruptured hose or failed pump, the switch is grounded and the electric assist pump will operate.
Booster units may be repaired by overhaul or replacement.
131
In a hydromax power brake booster, what happens if flow from the hydraulic pump is interrupted?
An electric pump backs up the system.
132
What should you do when replacing hydraulic hoses in the booster system?
The lines must conform must conform to the Society of Automotive Engineers (SAE) 1189 standard.
133
For diagnosis of most Hydraulic boosters failures that occur, what should you use?
Hydraulic boosters have troubleshooting charts that should be followed for accurate diagnosis.
134
What could be the cause if a power brake assist problem occurs?
If a power brake assist problem occurs, a faulty pump could be the cause.
Then this is suspected, pump flow and pressure tests should be performed.
135
How would you check brake boost?
To check brake boost, a pressure gauge capable of measuring at least 2000 psi can be installed at a caliper or wheel cylinder.
With the engine off, the reserve application pressure (electric pump) should be approximately 900 psi.
The electric pump should be audible during a reserve brake assist.
136
What should be the brake application PSI , when the engine is running and the power assist pump operating?
Brake application pressure should be approximately 1800 psi.
137
What is booster fluid?
Booster fluid is usually power steering fluid or automatic transmission fluid. It is necessary that the correct fluid is installed.
138
What can contamination with incorrect fluid cause?
Contamination with the incorrect fluid can cause failure of the system
139
What can a sticking control brake booster valve cause?
A sticking control valve can cause erratic booster performance.
140
What can restricted lines or filters cause for a power brake assist system?
low pressure, which can result in a hard pedal concern from the operator.
141
What can inaccurate stop light switch adjustment cause?
Inaccurate stop light switch adjustment may cause improper stoplight operation.
142
What is available to the ignition switch and emergency light circuits with the ignition switch off?
voltage is available to the ignition switch and emergency light circuits.
143
In some stoplight circuits, where is voltage supplied?
voltage is supplied to the brake light switch from the battery positive terminal.
144
What happens to the stoplight switch when the brakes are applied on a hydraulic brake system?
When the brakes are applied on a hydraulic brake system, brake pedal movement closes the stoplight switch.
145
What will worn pedal assemblies do?
Worn pedal assemblies can allow the brake light switch to close and the brake lights to activate.
This will allow the brake lights to drain the batteries while the truck sits.
146
What should you do to bulb contacts before installation?
Bulb contacts should be coated with electrical contact grease before installation.
147
What should you inspect wiring an connectors for?
Wiring and connectors should be inspected for corrosion.
148
What should you do when diagnosing dim stoplights?
When diagnosing dim stoplights, the technician will need to check for correct voltage to the bulb while the bulb is installed and operating.
