Which combination best reduces rollover risk in a combination vehicle?
Correct answer: Keep cargo low and slow down for turns
A low, centered load and reduced turn speed keep the center of gravity controlled.
Source: Texas DL-7C, March 2026, section 6, PDF page 91: Rollover prevention ↗
What can the crack-the-whip effect do during a sudden lane change?
Correct answer: Cause the rear trailer to overturn
Rearward amplification makes steering movement more severe toward the back of a multi-unit combination.
Source: Texas DL-7C, March 2026, section 6, PDF page 91: Rearward amplification ↗
Why can an empty combination vehicle be harder to stop smoothly than a fully loaded one?
Correct answer: Stiff suspension and strong brakes can reduce traction and lock wheels
Light loads can provide poor traction, making wheel lock, trailer swing, and jackknife more likely.
Source: Texas DL-7C, March 2026, section 6, PDF page 91: Stopping ↗
What should a driver do first if a low-clearance combination becomes stuck on railroad tracks?
Correct answer: Leave the vehicle and move away from the tracks
Occupants should get away from the tracks, then use the crossing information to contact emergency services.
Source: Texas DL-7C, March 2026, section 6, PDF page 92: Railroad crossings ↗
What is the earliest reliable sign that a trailer has started to skid?
Correct answer: The trailer begins moving out of line in the mirrors
Frequent mirror checks during hard braking reveal trailer swing before a jackknife develops.
Source: Texas DL-7C, March 2026, section 6, PDF page 92: Trailer skids ↗
How should a driver respond when trailer wheels lock and the trailer starts to swing?
Correct answer: Release the brakes to restore traction
The locked trailer brakes caused the skid; releasing them lets the tires regain grip.
Source: Texas DL-7C, March 2026, section 6, PDF page 93: Trailer skids ↗
What does off-tracking mean in a turn?
Correct answer: Rear wheels follow a different, tighter path than front wheels
Long vehicles’ rear wheels cut inside the path of their front wheels, with the last trailer cutting in most.
Source: Texas DL-7C, March 2026, section 6, PDF page 93: Off-tracking ↗
When backing a trailer on a curved path, which side is preferred?
Correct answer: The driver’s side
Driver-side backing provides the clearest view; straight-line backing is preferable whenever possible.
Source: Texas DL-7C, March 2026, section 6, PDF page 93: Backing ↗
What is the proper routine use of a trailer hand valve?
Correct answer: Testing the trailer brakes
The hand valve is for testing trailer brakes; normal braking should use the foot pedal.
Source: Texas DL-7C, March 2026, section 6, PDF page 94: Trailer hand valve ↗
What is the tractor protection valve designed to do?
Correct answer: Keep air in the tractor if the trailer breaks away or develops a major leak
The valve closes to protect the tractor air supply when trailer-side pressure is lost.
Source: Texas DL-7C, March 2026, section 6, PDF page 95: Tractor protection ↗
On a newer tractor, what does pulling out the red eight-sided trailer-air-supply knob do?
Correct answer: Shuts off trailer air and applies trailer emergency brakes
Pulling the red knob closes the supply; pushing it in supplies air to the trailer.
Source: Texas DL-7C, March 2026, section 6, PDF page 95: Trailer air supply ↗
What are the two air lines on a combination vehicle?
Correct answer: Service and emergency
The service line carries brake-control pressure, while the emergency line supplies tanks and controls emergency braking.
Source: Texas DL-7C, March 2026, section 6, PDF page 95: Air lines ↗
What happens when pressure is lost from the trailer emergency line?
Correct answer: Trailer emergency brakes apply and the tractor protection valve closes
Loss of supply-line pressure triggers trailer emergency braking and protects the tractor air supply.
Source: Texas DL-7C, March 2026, section 6, PDF page 95: Air lines ↗
Which color normally identifies the trailer emergency air line?
Correct answer: Red
Emergency or supply lines are normally red; service lines are normally blue.
Source: Texas DL-7C, March 2026, section 6, PDF page 95: Glad hands ↗
What danger can result from crossing the trailer service and emergency air lines?
Correct answer: The trailer tanks may not charge and trailer brakes may not work
Crossed connections may prevent spring-brake release or, on older trailers, allow movement without trailer brakes.
Source: Texas DL-7C, March 2026, section 6, PDF page 96: Glad hands ↗
How should trailer air-line shut-off valves be positioned in a multi-trailer combination?
Correct answer: Open except at the rear of the last trailer, where they are closed
Air must flow through every unit, while the unused line ends on the last trailer remain closed.
Source: Texas DL-7C, March 2026, section 6, PDF page 96: Shut-off valves ↗
Why must wheel chocks be used when parking an older trailer without spring brakes?
Correct answer: Stored air can leak away and release its emergency brakes
Air-operated emergency brakes on such trailers hold only while stored pressure remains.
Source: Texas DL-7C, March 2026, section 6, PDF page 96: Trailer parking ↗
Where is a trailer’s yellow ABS malfunction lamp normally located?
Correct answer: On the left side near a front or rear corner
Trailers equipped with ABS normally display the yellow fault lamp on the left side.
Source: Texas DL-7C, March 2026, section 6, PDF page 97: Trailer ABS ↗
Why must the fifth-wheel plate be properly lubricated before coupling?
Correct answer: Excess friction can create steering problems
The tractor and trailer must articulate freely across the fifth-wheel surface.
Source: Texas DL-7C, March 2026, section 6, PDF page 97: Fifth-wheel inspection ↗
How should the trailer height be set before backing fully under it?
Correct answer: Low enough to be raised slightly by the tractor
A slightly low trailer contacts the fifth-wheel correctly; excessive height can cause a false coupling.
Source: Texas DL-7C, March 2026, section 6, PDF page 98: Coupling ↗
