You pump down the brakes to test the trailer supply valve, and the knob pops out at the reading shown. Is that acceptable?
What to know for the Doubles and Triples test
Each set draws 20 of our 32 questions
The doubles and triples test earns the T endorsement, which you need to pull two or three trailers. It is a knowledge test only, with no separate skills test. It covers hookup, trailer order, stability, inspection and air brake checks. A passing score is 80 percent under 49 CFR 383.135.
Last checked against the handbook and 49 CFR on .
What the T endorsement requires
Federal rules require knowledge of hookup procedures, where the heaviest trailer goes, handling and stability traits, traffic problems the rig creates, and general operating practice. 49 CFR 383.115
The T endorsement needs a knowledge test only; no skills test is required for it. 49 CFR 383.93
To keep the rig stable, hitch the more heavily loaded trailer directly behind the tractor and put the lighter trailer at the back. CDL Handbook 7.2
Stability and rollover
Crack-the-whip makes a multi-trailer rig tip over more readily than other combinations, and the last trailer is the likeliest to go. CDL Handbook 7.1
Rearward amplification for a triple is about 3.5, so its last trailer rolls roughly 3.5 times as easily as the rear of a five-axle tractor-semitrailer. CDL Handbook 6.1
A speed that is safe on a curve for a straight truck or a single trailer can be too fast for doubles or triples, so slow down before ramps, curves and corners. CDL Handbook 7.1
Look far ahead and move smoothly, because the rig must change lanes or slow down gradually to avoid a rollover or jackknife. CDL Handbook 7.1
Handling problems the federal rule names
Topic
What to know
Off-tracking
Rear wheels cut inside the path of the front wheels
Response to steering
Sudden steering inputs grow toward the back of the rig
A converter dolly has one or two axles and a fifth wheel, and it lets a second semitrailer be hitched behind the first to make a double. CDL Handbook 7.2
If the rear trailer lacks spring brakes, connect the emergency line, fill its tank, then disconnect the line so the emergency brakes apply; chock the wheels if you doubt the brakes. CDL Handbook 7.2
The rear trailer should sit slightly lower than the dolly fifth wheel so it is lifted a little as the dolly slides under. CDL Handbook 7.2
After backing under, raise the landing gear, tug against the kingpin, and check that no gap shows between the two fifth-wheel plates. CDL Handbook 7.2
When uncoupling, drag the dolly clear of the rear trailer before you unlatch the pintle hook, because the tow bar can fly up and injure you. CDL Handbook 7.2
To uncouple triples, remove the third trailer first as you would with a double, then treat the rest as a standard double. CDL Handbook 7.2
Shut-off valves and air checks
Valve and test positions for doubles and triples
Item
Correct position or result
Service and emergency valves, tail of each front trailer
Open
Service and emergency valves, tail of the last trailer
Closed
Converter dolly tank drain valve while driving
Closed
Tractor protection valve control pops out
Usually between 20 and 45 psi
Trailer emergency brakes test
Pull the supply control out and tug gently; the trailer should hold
Trailer service brakes test
Roll slowly forward and apply the hand valve; brakes should come on
To confirm air reaches every trailer, push in the supply knob, open the emergency line valve at the very back and listen for escaping air, then do the same with the service line while a brake is applied. CDL Handbook 7.4
If no air escapes at the rear, a shut-off valve between units is probably closed, and the rear brakes will not work until you open it. CDL Handbook 7.4
Inspect the dolly and pintle hook too: the pintle eye must sit in the hook, the hook must be latched, and the safety chains and light cords must be connected. CDL Handbook 7.3
Dollies manufactured from 1 March 1998 onward must carry ABS, shown by a yellow lamp on the left side. CDL Handbook 7.1
Driving habits
Doubles and triples need larger gaps and longer following distance, because they cannot stop or turn suddenly. CDL Handbook 7.1
Doubles and Triples test: common questions
Is there a skills test for the doubles and triples endorsement?
No. The T endorsement requires a knowledge test only. Federal rules list a knowledge test for double and triple trailers, in contrast with passenger and school bus endorsements, which also need a skills test. 49 CFR 383.93
Which trailer goes first in a set of doubles?
The more heavily loaded semitrailer goes directly behind the tractor, and the lighter trailer goes at the rear. This gives the rig better stability. CDL Handbook 7.2
Which trailer is most likely to roll over?
The last trailer. Each trailer adds to the whip effect, so the final one swings the most in a quick lane change. For a triple, the last trailer rolls about 3.5 times as easily as a five-axle tractor-semitrailer. CDL Handbook 7.1
Should the rear shut-off valves be open or closed?
Open on every trailer except the last. Those at the tail of the final trailer must be closed, or air would leak out of the system. CDL Handbook 7.3
What is a converter dolly?
A coupling device with one or two axles and a fifth wheel. Hitched behind the first trailer, it lets a second semitrailer be coupled to the rig, forming a double. CDL Handbook 7.2