During a safe, low-speed service-brake check, the vehicle pulls to one side and the response feels delayed. What should you conclude?
Correct answer: A braking problem needs investigation and correction.
Pulling, delayed response or unusual feel can reveal trouble. The check identifies a symptom, not one certain component failure.
Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗
What extra part of stopping distance must an air-brake driver allow for, beyond perception, reaction and wheel-braking distance?
Correct answer: Distance traveled while the air brakes respond to the application.
Air pressure takes time to reach and operate the brakes. The distance traveled during that delay adds to stopping distance and varies with speed and conditions.
Source: South Carolina CDL Manual, body March 2025, section 5.4.4; printed 5-8; physical PDF page 94; reviewed September 21, 2026. ↗
The handbook warns against applying parking brakes when they are very hot. What must a driver still ensure while following the cooling procedure?
Correct answer: The vehicle is securely prevented from rolling, including suitable chocks.
The hot-brake caution is not permission to leave a vehicle unsecured. Use a level location, appropriate chocks and the vehicle’s safe parking procedure while the brakes cool.
Source: South Carolina CDL Manual, body March 2025, section 5.4.8; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗
Under the cited current FMVSS 121 requirement, ignition is in RUN and service-reservoir pressure is below 60 psi. What must the warning signal do?
Correct answer: Give a continuous low-air warning.
S5.1.5 requires a continuous signal other than the gauge below 60 psi with ignition in RUN. It may be visible, or both audible and visible. Applicability depends on the vehicle and the standard in effect when manufactured.
Source: 49 CFR 571.121, S5.1.5; eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
For a non-exempt air-braked power unit not subject to FMVSS 121 when manufactured, the governor cuts out at 100 psi. Under 49 CFR 393.51(c)(2), at what pressure and below is continuous warning required?
Correct answer: 50 psi, the lower of 55 psi and half of cut-out pressure.
This particular older-vehicle rule uses 55 psi or half the governor cut-out pressure, whichever is less. Half of 100 is 50; do not use this exception as the warning threshold for modern FMVSS 121 vehicles.
Source: 49 CFR 393.51(c)(2), with paragraph (f) exceptions excluded by the question; eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
Under the cited FMVSS 121 gauge requirement, what pressure information must be readily visible from the normal driving position?
Correct answer: The reservoir pressure in each service-brake system.
S5.1.4 requires a pressure gauge in each service-brake system. The reading is reservoir pressure, and the separate low-air warning is not replaced by the gauge.
Source: 49 CFR 571.121, S5.1.4; eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
Under federal commercial-vehicle brake equipment rules, how must corresponding brake chambers on the two ends of an axle compare?
Correct answer: The service chambers must match in size, and the spring chambers must match in size.
49 CFR 393.47(b) requires matching service-brake chamber sizes across the axle and matching spring-brake chamber sizes across the axle. It does not say a service chamber and spring chamber must be identical to each other.
Source: 49 CFR 393.47(b); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
Which minimum thickness must commercial-vehicle brake drums and rotors meet under 49 CFR 393.47(g)?
Correct answer: The limits established by the drum or rotor manufacturer.
The rule uses the drum or rotor manufacturer’s limits. Do not replace that standard with a guessed universal dimension or a visual impression that the part still looks usable.
Source: 49 CFR 393.47(g); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗
Which part of the air-brake system is used for ordinary stops made with the brake pedal?
Correct answer: The service brake system.
The manual distinguishes service, parking and emergency braking. Ordinary pedal-controlled stops use the service brake system; the parking control serves a different function.
Source: South Carolina CDL Manual, body March 2025, section 5 (introduction); printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗
How does the CDL manual describe the emergency brake system on an air-braked vehicle?
Correct answer: It uses parts of the service and parking systems during a brake-system failure.
The emergency function draws on parts of the service and parking brake systems to help stop during a system failure. The warning devices alert the driver but do not themselves provide braking force.
Source: South Carolina CDL Manual, body March 2025, section 5 (introduction); printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗
In the ordinary governor example in South Carolina Section 5, approximately which cut-in and cut-out pressures are given, subject to manufacturer specifications?
Correct answer: Cut in at 100 psi and cut out at 125 psi.
The manual uses about 100 psi for cut-in and 125 psi for cut-out and instructs the driver to check manufacturer specifications. These example settings are not the spring-brake application range or a universal value for every vehicle.
Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗
At approximately what pressure does the South Carolina manual say the air-tank safety relief valve is usually set to open?
Correct answer: 150 psi.
The handbook gives 150 psi as the usual safety-valve setting. The valve protects against excessive pressure; air being released by it calls for a mechanic to correct the fault, not treating it as the normal governor cut-out.
Source: South Carolina CDL Manual, body March 2025, section 5.1.6; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗
For the dual-air-system procedure in South Carolina Section 5.2, what minimum buildup does the manual specify in both primary and secondary systems before driving?
Correct answer: 100 psi in each system.
Section 5.2 calls for at least 100 psi in both systems before driving. The low-air warning going out or one gauge rising is not the same as both circuits reaching the required pressure; also follow the vehicle manufacturer’s procedure.
Source: South Carolina CDL Manual, body March 2025, section 5.2; printed 5-5; physical PDF page 91; reviewed September 21, 2026. ↗
A secured single vehicle has its engine off, parking brake released and service brake released for the manual’s released-brake leakage check. A one-minute drop is 2.5 psi. How does that compare with the stated criterion?
Correct answer: It fails the released-brake criterion of less than 2 psi per minute.
The released-brake check uses less than 2 psi per minute for a single vehicle. The separate applied-brake check allows no more than 3 psi; do not substitute its limit for this setup. Secure the vehicle against rolling before releasing parking brakes.
Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗
A secured truck-and-one-trailer combination loses 3.5 psi in one minute with the engine off and the brakes released during the manual’s released-brake leakage check. Which conclusion fits that setup?
Correct answer: It fails the released-brake criterion of less than 3 psi per minute.
The manual gives less than 3 psi per minute for the released-brake combination check. The 4-psi figure belongs to the distinct fully applied check. Rollaway protection is essential during either inspection setup.
Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗
Why does the CDL manual give a special parking precaution when brakes are very wet and temperatures are freezing?
Correct answer: Applied wet brakes can freeze in place and prevent the vehicle from moving.
Very wet brakes can freeze if applied for parking in freezing conditions. The manual calls for suitable wheel chocks on a level surface and describes safely drying wet brakes. The exception is not permission to leave a vehicle unsecured.
Source: South Carolina CDL Manual, body March 2025, section 5.4.8; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗
What can excessive heat do to a drum brake that contributes to brake fade?
Correct answer: Expand the drum and reduce effective friction from the lining.
The manual describes both heat-related loss of lining friction and drum expansion. Shoes must travel farther as the drum expands, reducing effective braking; continued overuse can leave the vehicle unable to slow or stop.
Source: South Carolina CDL Manual, body March 2025, section 5.4.5; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗
A driver assumes ABS guarantees a shorter stop on every surface. What does the air-brake section actually say?
Correct answer: ABS helps control, but does not necessarily shorten stopping distance.
ABS helps prevent wheel lockup and maintain control. The manual explicitly warns that it does not necessarily reduce stopping distance, so the driver must still allow an appropriate following and stopping margin.
Source: South Carolina CDL Manual, body March 2025, section 5.1.16; printed 5-4; physical PDF page 90; reviewed September 21, 2026. ↗
Where does the South Carolina CDL manual tell a driver to look for a trailer’s yellow ABS malfunction lamp?
Correct answer: On the trailer’s left side, near the front or rear corner.
The manual places the trailer ABS malfunction lamp on the left side at a front or rear corner. This is distinct from the instrument-panel lamps on the tractor, truck or bus.
Source: South Carolina CDL Manual, body March 2025, section 5.1.16; printed 5-4; physical PDF page 90; reviewed September 21, 2026. ↗
An ABS malfunction is detected, but there is no separate evidence that the regular service brakes have failed. What distinction should the driver understand?
Correct answer: Regular braking can remain available, but ABS needs prompt service.
ABS is additional control equipment, not a replacement for the ordinary brakes. The manual says normal brake functions can remain when ABS fails and the system should be serviced soon. A separate braking fault or low-air warning still requires its own safe response.
Source: South Carolina CDL Manual, body March 2025, section 5.4.2; printed 5-8; physical PDF page 94; reviewed September 21, 2026. ↗
