Which tractor-trailer condition particularly increases the chance of getting hung up on a raised railroad crossing?
Correct answer: Low clearance under the trailer or its landing gear.
A raised crossing can catch low-slung equipment or landing gear. Check the route and clearance before entering; once stuck, get clear of the tracks and seek emergency help.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.1; railroad-highway crossings; Low-clearance rail crossing; physical PDF page 174 (printed 174); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
You are driving a tractor without its semitrailer. What should you remember about braking?
Correct answer: Low weight on the rear drive axles can make smooth stopping more difficult.
A bobtail tractor can have poor drive-axle traction and be difficult to stop smoothly. Allow adequate space; actual stopping performance depends on the vehicle, brakes, load and road.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.1.3; Bobtail braking; physical PDF page 173 (printed 173); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
The trailer wheels have locked during braking and the trailer begins to swing. What action addresses the cause of this trailer skid?
Correct answer: Release the braking that caused the lockup so the trailer wheels can regain traction.
A locked trailer wheel cannot provide normal directional grip. More trailer braking worsens this skid. This describes an actual wheel-lock trailer skid, not a general instruction to pump functioning ABS.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.1.5; Trailer-wheel skid; physical PDF page 175 (printed 175); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
A curved backing maneuver with a semitrailer cannot be avoided. Which side does the handbook favor when a safe setup permits it?
Correct answer: Back toward the driver’s side for better visibility.
Driver-side backing generally gives a better view of the trailer and path. Straight backing is preferable when possible; inspect the area and stop to look again whenever uncertain.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.1.7; Backing visibility; physical PDF page 177 (printed 177); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
During ordinary driving, which control should normally apply the tractor and trailer service brakes together?
Correct answer: The foot brake pedal.
The foot pedal controls service braking throughout the combination. The trailer hand valve is for checking trailer brakes, not routine driving or straightening a skid.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.1; Trailer hand valve in traffic; physical PDF page 179 (printed 179); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Why is the trailer hand valve unsuitable as a parking brake?
Correct answer: Air can leak away and release the brakes it applied.
A hand-valve application depends on air pressure and is not a secure parking method. Use the correct parking system and suitable chocks, especially for trailers without spring brakes.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.1; Hand valve is not parking; physical PDF page 179 (printed 179); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
A trailer air hose tears apart. What is the purpose of the tractor protection valve?
Correct answer: Protect the tractor’s braking-air supply from escaping through the trailer system.
The protection system limits the trailer failure’s effect on tractor air. It cannot repair the damaged connection or authorize continued operation of an unsafe combination.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.2; Tractor air protection; physical PDF page 179 (printed 179); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
On a newer tractor with the standard red eight-sided trailer-air control, what does pushing it in do?
Correct answer: Supplies air to the trailer system.
The red control is pushed in to supply trailer air and pulled out to shut off supply and activate trailer emergency braking. Know any different controls on the actual vehicle.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.3; Red trailer-supply control; physical PDF page 180 (printed 180); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
What does the air pressure in the trailer service line communicate?
Correct answer: The amount of braking requested by the pedal or hand control.
The service or control line carries the braking signal to relay valves. It is not the line whose principal job is charging the trailer reservoirs.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.4; Service-line signal; physical PDF page 180 (printed 180); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Which pair of functions belongs to the trailer emergency or supply air line?
Correct answer: Charge the trailer tanks and control emergency braking when supply pressure is lost.
The supply line fills trailer reservoirs. Loss of its pressure also activates the trailer emergency-brake system; it has a different job from the service signal line.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.4; Supply-line functions; physical PDF page 180 (printed 180); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
How do relay valves help the trailer service brakes respond promptly?
Correct answer: They deliver air from nearby trailer reservoirs in response to the service-line signal.
The control signal tells relay valves how much reservoir air to send to the trailer brakes. Supplying that air locally speeds the response.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.6; Trailer relay valves; physical PDF page 182 (printed 182); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
When standard red and blue air-line markings are used, which connection is correct?
Correct answer: Red to emergency/supply and blue to service.
The usual color code separates the red supply line from the blue service line. Verify markings and the vehicle’s connections rather than assuming every hose is correctly identified.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; 6.2.4; Identifying air connections; physical PDF page 180 (printed 180); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
