How should rear air-line shut-off valves be set in a doubles combination?
Correct answer: Open on the lead trailer and closed on the last trailer
Air must flow through intermediate units, while unused ends at the last trailer stay closed.
Source: Texas DL-7C, March 2026, section 7, PDF page 107: Shut-off valves ↗
Which converter-dolly items require a secure inspection?
Correct answer: Pintle eye and hook, safety chains, air lines, and light cords
All dolly connections must be latched, supported, connected, and undamaged.
Source: Texas DL-7C, March 2026, section 7, PDF page 108: Dolly inspection ↗
How is emergency-line airflow verified at the back of a doubles rig?
Correct answer: Charge the system and briefly open the last trailer’s emergency shut-off to hear escaping air
Secure the rig with the tractor parking brake and/or wheel chocks, charge the system, then briefly open and close the last emergency-line valve. Escaping air shows supply reaches the rear. Check service-line flow separately, and close each valve after testing.
Source: Texas DL-7C, March 2026, section 7, PDF page 108: Air-flow test ↗
At approximately what pressure range does the trailer-air-supply control usually pop out during its test?
Correct answer: 20 to 45 psi
The exact value follows the manufacturer, but the handbook gives a usual 20-to-45-psi range.
Source: Texas DL-7C, March 2026, section 7, PDF page 108: Tractor protection test ↗
On a combination equipped with a trailer hand control, how should the trailer service brakes be tested?
Correct answer: Move forward slowly and apply the trailer hand control to feel the brakes engage
At normal air pressure with parking brakes released, move forward slowly and apply the hand control, if equipped. Feel the trailer brakes engage. Use the foot pedal for normal braking, not the hand control.
Source: Texas DL-7C, March 2026, section 7, PDF page 108: Trailer brake tests ↗
