Why must the dolly be pulled clear of the rear trailer before its pintle connection to the lead trailer is released?
Correct answer: The loaded dolly tow bar could fly upward if released while still under the trailer.
Releasing the pintle too early can make the dolly tongue kick up violently. Follow the full uncoupling order and keep people clear of the coupling area.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.2; Dolly tongue kick-up; physical PDF page 204 (printed 204); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
In a jurisdiction permitting triples: Using the handbook’s common triple-trailer procedure, which trailer is uncoupled first?
Correct answer: The third trailer, followed by the remainder as a doubles rig.
The handbook describes removing the third trailer using the doubles method, then uncoupling the remaining rig. Other equipment requires its own approved procedure. Maryland does not permit triple-trailer highway operation.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.3; Triple uncoupling order; physical PDF page 204 (printed 204); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Once the dolly is clear of the rear trailer, what must be done before releasing the pintle hook on the lead trailer?
Correct answer: Lower the dolly support and apply its spring brakes or chock its wheels.
Support and secure the dolly before disconnecting it. The handbook also calls for disconnecting its safety chains in the proper sequence.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.2; Dolly security before unhitching; physical PDF page 204 (printed 204); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
A spare tire is carried on the converter gear. What must be checked before departure?
Correct answer: It is securely retained on the dolly.
The spare is part of the load carried on the dolly and must be secured. It must not damage or obstruct air lines, electrical connections or safety equipment.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Dolly spare tire; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
During a correctly secured air-flow check, no air escapes from the opened supply-line valve at the back of the last trailer. What should you investigate?
Correct answer: A closed intermediate shut-off or another fault preventing air reaching the rear.
Air must reach every connected trailer. Check intermediate valves and diagnose the problem before driving; a charged tractor gauge alone does not prove rear-trailer supply.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.4.1; Missing supply air at the rear; physical PDF page 207 (printed 207); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After confirming air at the last trailer’s service and supply outlets during the secured test, what should you do with those rear valves?
Correct answer: Close them again before normal operation.
The outlets are opened briefly for the prescribed check. They must be closed again because there is no following trailer to receive that air.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.4.1; Restoring valves after an air-flow test; physical PDF page 207 (printed 207); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
For a modern full-trailer hitch outside driveaway-towaway operations, where should the required safety device attach on the towing vehicle?
Correct answer: To the frame or its extension, independently of the pintle hook or tow-bar attachment device.
The safety connection must remain effective if the tow-bar attachment fails. This question uses modern equipment; the rule has a specific limited exception for certain pre-July 1973 forgings or castings.
Source: 49 CFR 393.70(d), (d)(1); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
How much slack should a full-trailer safety device have under the cited federal coupling rule?
Correct answer: No more than is needed for the vehicles to turn properly.
Allow necessary turning movement without unnecessary slack. The safety device must also be arranged to keep a failed or disconnected tow bar off the ground.
Source: 49 CFR 393.70(d)(2), (4); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
Under the federal rule for full trailers and converter dollies, the required safety device and its attachments must have what minimum ultimate strength?
Correct answer: At least the gross weight of the vehicle or vehicles being towed.
The specified ultimate strength is based on the gross weight being towed. This is not the working-load-limit calculation used for cargo tiedowns.
Source: 49 CFR 393.70(d)(3); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
A converter dolly has the solid tongue and no-swivel arrangement specified in 49 CFR 393.70(d)(6). If it uses a single safety chain alone, how must that device be aligned?
Correct answer: Along the centerline of the trailer tongue.
The rule permits a single device for that specified dolly configuration and requires it to be in line with the tongue’s centerline. Do not generalize this exception to every trailer or hinged dolly.
Source: 49 CFR 393.70(d)(6)(i)–(iii); Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
Outside driveaway-towaway operations, an air-braked converter dolly was manufactured on March 1, 1998. What does the cited federal rule require?
Correct answer: An antilock braking system meeting the applicable FMVSS 121 requirements.
The relevant requirement applies to air-braked vehicles other than truck tractors manufactured on or after March 1, 1998. Later malfunction-signal circuit dates are a separate issue.
Source: 49 CFR 393.55(c)(2); see (e) for the exterior malfunction indicator; Official eCFR XML dated September 11, 2026 checked against the linked rule; reviewed September 15, 2026. ↗
What must a driver seeking Maryland’s T endorsement understand about testing and triple-trailer operation?
Correct answer: Pass the endorsement knowledge test; it does not authorize triples on Maryland highways.
Maryland’s manual requires the T-endorsement knowledge test but states that triples are not allowed in Maryland. Transportation §24-104.1(l) prohibits a truck tractor on a State highway in combination with more than two trailers or semitrailers, regardless of total length. An endorsement does not override that restriction.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7 introduction; physical PDF page 200 (printed 200); Maryland Transportation §24-104.1(l), official General Assembly text checked; 49 CFR 383.93(c)(1), official eCFR XML dated September 11, 2026; reviewed September 15, 2026. ↗
