Practice question Why check air all the way to the back? Only tractor brakes need airAll trailer brakes need air to workIt measures fuel economyIt proves landing gear is up Answer: B — All trailer brakes need air to work The correct answer is shown above. Try the full quiz for additional practice. Practice more: Washington CDL Doubles and Triples Endorsement Test Practice Questions - Washington CDL Doubles and Triples Endorsement Test Exam →