The questions emphasize what the driver should observe and why proper pressure, adjustment, heat control, and traction matter.
01Leak checks
02Applied test
03ABS
04Brake lag
05Downgrades
06Brake fade
PRACTICE MODE
Air Brakes Test 2
Questions appear in a new order on each attempt. Choose one answer, review the explanation, and use Back or Next to move through the test.
Question 1 of 25Score 0
During an applied leakage test, what loss is generally allowed for a combination vehicle?
Answer: No more than 4 psi in one minute Under the standard applied-test setup, a combination vehicle should generally lose no more than 4 psi in one minute. Greater loss signals a dangerous leak. Study: Air Brakes — applied leakage test
For many dual air systems, how quickly should pressure build from 85 to 100 psi?
Answer: Within 45 seconds Many modern dual systems should build from 85 to 100 psi within 45 seconds at governed engine speed. Check the manufacturer and state manual because procedures can differ for older vehicles. Study: Air Brakes — pressure buildup
What are common governor cut-in and cut-out pressures?
Answer: About 100 psi cut-in and 125 psi cut-out Typical systems resume compressor loading near 100 psi and stop near 125 psi. Exact specifications vary, so abnormal readings should be checked against the vehicle documentation. Study: Air Brakes — governor check
Why do air brakes add lag to stopping distance?
Answer: Air takes time to travel through the lines Air-brake activation is not instantaneous; at highway speed, the roughly half-second lag adds travel before braking begins. Total stopping distance also includes perception, reaction, and braking distance. Study: Air Brakes — brake lag
What is controlled braking?
Answer: Applying brakes as hard as possible without locking the wheels Controlled braking seeks maximum braking while maintaining wheel rotation and steering control. If wheels lock, release enough pressure to regain control. Study: Air Brakes — emergency braking
When is stab braking described as an emergency technique?
Answer: For vehicles without ABS Stab braking means fully applying the brakes, releasing when wheels lock, and reapplying after they roll. CDL manuals distinguish it from ABS braking and describe it for vehicles without ABS. Study: Air Brakes — emergency braking
How should a driver brake in an ABS-equipped vehicle during an emergency?
Answer: Apply firm pressure and hold it while steering as needed ABS automatically manages wheel lock, so the driver should brake firmly without pumping. ABS does not compensate for excessive speed or poor following distance. Study: Air Brakes — ABS
What does an ABS warning lamp that stays on after its check indicate?
Answer: ABS may be malfunctioning The lamp should illuminate for a bulb check and then go out according to the system design. If it stays on, normal brakes may still work, but ABS needs service. Study: Air Brakes — ABS warning lamp
Before starting down a long grade, what should an air-brake driver do?
Answer: Select a safe low gear and establish a safe speed Gear selection and speed control must happen before the descent. Engine braking should carry much of the work while service brakes control speed. Study: Air Brakes — downgrades
On a long downgrade, what is the role of the service brakes?
Answer: Supplement the engine’s braking effect A low gear provides primary retarding force, while intermittent correct service-brake use controls speed. Riding the brakes can create dangerous heat and fade. Study: Air Brakes — downgrades
What commonly causes air-brake fade?
Answer: Excessive heat from overuse Overheated drums expand and linings lose effectiveness, reducing stopping power. Safe gear selection and proper braking technique limit heat buildup. Study: Air Brakes — brake fade
Why is repeated light pumping or “fanning” of the brakes dangerous while driving?
Answer: It wastes air pressure and can overheat brakes Unnecessary applications consume stored air and may build heat. Use smooth, planned braking and monitor system pressure. Study: Air Brakes — brake use
What is the basic “snub braking” idea on a downgrade?
Answer: Brake enough to reduce speed below the safe speed, then release With the proper low gear selected, apply the service brakes firmly to reduce speed by about 5 mph, then release while the vehicle gradually returns to the safe speed. Follow the current manual’s procedure. Study: Air Brakes — downgrade technique
Why should parking brakes generally not be set when brakes are extremely hot?
Answer: The drums can be damaged as they cool Hot brake drums can be damaged by pressure from the parking brakes as they cool. If safe, use wheel chocks and allow the brakes to cool according to operating guidance. Study: Air Brakes — hot brakes
How can wet brakes be dried after a water crossing?
Answer: Apply light pressure while moving slowly Gentle application creates heat and helps dry friction surfaces while the driver checks for even braking. Keep speed low until normal braking returns. Study: Air Brakes — wet brakes
What should a driver do when the low-air warning activates while driving?
Answer: Stop safely as soon as possible and correct the problem The warning means pressure is becoming unsafe. Do not wait for spring brakes to apply automatically, because control can be lost. Study: Air Brakes — low pressure
In a combination vehicle, what does the service air line carry?
Answer: Air controlled by the foot brake or trailer hand valve The service line carries application pressure to the trailer brakes. It is commonly identified by blue fittings or markings. Study: Air Brakes — trailer service line
What is a primary function of the trailer emergency air line?
Answer: Supply air to trailer tanks and control emergency braking The emergency line supplies reservoir air and helps trigger trailer emergency brakes if supply pressure is lost. It is commonly identified by red fittings or markings. Study: Air Brakes — trailer emergency line
What does the tractor protection valve do?
Answer: Protect tractor air if the trailer breaks away or leaks badly The valve closes the trailer supply and helps preserve tractor air during a major trailer-system loss. It works with the trailer air-supply control. Study: Air Brakes — tractor protection
Which color is commonly used for the trailer air-supply control?
Answer: Red The trailer air-supply knob is commonly red and eight-sided. Pushing it in supplies air; pulling it out closes the supply and applies trailer emergency brakes. Study: Air Brakes — controls
Which color commonly identifies the trailer service line?
Answer: Blue The service line is commonly blue, while the emergency or supply line is red. Correct connections are essential for proper trailer brake operation. Study: Air Brakes — gladhands
What force holds spring brakes released during normal operation?
Answer: Air pressure Compressed air overcomes the spring force and releases the brakes. When pressure falls too far, the springs can apply the brakes mechanically. Study: Air Brakes — spring brakes
Why must compressed air be treated with caution during inspection?
Answer: It stores enough energy to cause injury Pressurized components, lines, and springs can release energy suddenly. Follow safe procedures and leave repairs to qualified personnel. Study: Air Brakes — inspection safety
If an air tank has a manual drain, when should it normally be drained?
Answer: At the end of each driving day Daily draining removes moisture and oil that can damage components or freeze. Automatic drains still require inspection for proper operation. Study: Air Brakes — reservoir draining
Why can water in an air-brake system be especially dangerous in winter?
Answer: It can freeze and block air flow Frozen moisture can obstruct lines and valves, reducing or preventing proper brake operation. Draining tanks and maintaining dryers or evaporators helps control moisture. Study: Air Brakes — cold weather
Answer from memory so your first score shows which topics need work.
02
Review the explanations
Return to the cited handbook section whenever an idea is unclear.
03
Retake after review
Aim for consistent mastery, not memorizing the position of an answer.
FACT-CHECKED STUDY BASIS
Official handbook sources
Questions are original study material based on the standard CDL manual topics used by state licensing agencies. Requirements and test formats can vary by state, so always confirm details in your current state CDL manual.
Important: This independent study site is not affiliated with FMCSA or any state DMV. It does not replace the official handbook or professional instruction.