CDL air brakes practice test 4

A: Release the brakes, then re-apply them as soon as you can. With controlled braking, if you need a larger steering adjustment or the wheels lock, release the brakes, then re-apply them as soon as you can. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.3 Emergency Stops, page 5-8.

D: A spring. Releasing the pedal lets the S-cam turn back, and a return spring draws the shoes off the drum so the wheels turn freely. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.8 Foundation Brakes, page 5-2.

C: Apply the brakes all the way, release when the wheels lock, and apply fully again as soon as the wheels start rolling. Stab braking: apply the brakes all the way, release them when the wheels lock up, and apply them fully again as soon as the wheels start rolling. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.3 Emergency Stops, page 5-8.

D: The brake chamber push rod pushes a wedge directly between the ends of two brake shoes. In wedge brakes the push rod drives a wedge between the ends of two shoes, shoving them apart and against the inside of the drum. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.8 Foundation Brakes, page 5-2.

C: Up to one second. After you let off, the wheels may need as long as one second to start turning again. Re-braking before they roll won't let the vehicle straighten out. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.3 Emergency Stops, page 5-8.

A: A power screw. Air disc brakes use a power screw turned by the brake chamber and slack adjuster; it clamps the rotor between the caliper's lining pads like a large C-clamp. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.8 Foundation Brakes, page 5-2.

C: The vehicle won't straighten out. Wait for the wheels to start rolling before re-applying; if you re-apply before they roll, the vehicle won't straighten out. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.3 Emergency Stops, page 5-8.

B: They may be self-adjusting or may need manual adjustment. The manual says wedge brakes may be self-adjusting or may require manual adjustment, and they are less common than S-cam brakes. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.8 Foundation Brakes, page 5-2.

D: Enough air remains in the air tanks. Controlled braking is possible only while enough air remains in the tanks, which is why you should stop and park safely as soon as the low air warning comes on. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.7 Low Air Pressure, page 5-9.

D: How much pressure is in the air tanks. Supply pressure gauges are connected to the air tanks and show how much pressure is in them. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.9 Supply Pressure Gauges, page 5-2.

B: The spring brakes do not work on all axles. A heavily loaded vehicle takes a long distance to stop on spring brakes because the spring brakes don't work on all axles. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.7 Low Air Pressure, page 5-9.

D: There is a gauge for each half of the system, or a single gauge with two needles. With a dual air system there is a gauge for each half of the system, or one gauge with two needles. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.9 Supply Pressure Gauges, page 5-2.

B: They may skid out of control. When the spring brakes grab, a lightly loaded vehicle, or any vehicle on a slippery road, may skid out of control. Stopping while you still have air for the foot brake is much safer. Commercial Driver's License Manual (CDL Driver's Manual): 5.4.7 Low Air Pressure, page 5-9.

D: How much air pressure you are applying to the brakes. This gauge reads the air pressure being sent to the brakes when you brake (some vehicles don't have one). The supply gauge shows tank pressure. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.10 Application Pressure Gauge, page 5-3.

B: Brakes out of adjustment, air leaks, or mechanical problems. The manual lists brakes out of adjustment, air leaks, and mechanical problems as other causes of needing more application pressure. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.10 Application Pressure Gauge, page 5-3.

B: Yes, it is required. Every air-braked vehicle must have a low air pressure warning signal. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.11 Low Air Pressure Warning, page 5-3.

C: 55 psi. The warning you can see must come on before tank pressure falls below 55 psi (or half the governor cut-out pressure on older vehicles). For vehicles not covered by FMVSS 121, 49 CFR 393.51(c) likewise requires a warning at 55 psi (or half the governor cut-out, whichever is less). Commercial Driver's License Manual (CDL Driver's Manual): 5.1.11 Low Air Pressure Warning, page 5-3.

A: A red light, possibly with a buzzer. The warning is usually a red light, and a buzzer may also come on. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.11 Low Air Pressure Warning, page 5-3.

B: A low air warning device that drops a mechanical arm into your view when pressure falls below 55 psi. A wig wag is a type of low air warning: it drops a mechanical arm into the driver's view when system pressure falls below 55 psi. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.11 Low Air Pressure Warning, page 5-3.

B: It must be put in the 'out of view' position by hand and won't stay there until pressure is above 55 psi. An automatic wig wag rises out of view when pressure goes above 55 psi. The manual-reset type has to be moved out of view by hand and won't stay there until pressure is above 55 psi. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.11 Low Air Pressure Warning, page 5-3.

C: 80-85 psi. The manual notes that on large buses it is common for the low pressure warning to signal at 80-85 psi. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.11 Low Air Pressure Warning, page 5-3.

D: An electric switch worked by air pressure turns them on. The air brake system uses an electric switch, worked by air pressure, to turn on the brake lights and warn drivers behind you. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.12 Stop Light Switch, page 5-3.

B: Some older vehicles made before 1975. The manual describes cab-controlled front brake limiting valves only on some vehicles built before 1975. Federal rule 49 CFR 393.48(b)(1) allows manual devices of this kind only on buses, trucks and tractors built before March 1, 1975. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.13 Front Brake Limiting Valve, page 5-3.

D: Only when the brakes are put on very hard (55 psi or more application pressure). Automatic limiting valves reduce air to the front brakes except when the brakes are applied very hard (55 psi or more application pressure). The driver can't control them. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.13 Front Brake Limiting Valve, page 5-3.

A: Because air pressure can eventually leak away. Air can slowly escape from the system, so parking and emergency brakes have to be held on by mechanical force. Spring brakes are the usual way to do this. Commercial Driver's License Manual (CDL Driver's Manual): 5.1.14 Spring Brakes, page 5-3.