Doubles Converter Dolly Air Line Failure at a Rural Intersection
You are pulling a set of doubles: a tractor, a 28-foot lead trailer, a converter dolly, and a 28-foot rear trailer. You stop at a rural stop sign on a two-lane road and prepare to turn left onto a highway. As you release the parking brake, you hear a sharp hiss and the low air pressure warning buzzer sounds. The tractor gauge drops from 110 psi to about 65 psi in a few seconds. The rear trailer's brakes do not appear to be holding, and the dolly's emergency spring brakes have not set. There is no traffic close by, but the intersection is on a slight downhill grade.
1. What is the most likely air system problem, and why are the rear trailer and dolly brakes not holding?
Read the worked answer
The sudden hiss and rapid pressure drop point to a broken or separated air line, most likely at the dolly or the rear trailer supply line. In a doubles set, the dolly carries the air and electrical connections between the lead and rear trailers. If the emergency (supply) line separates, the tractor's protection valve should close to preserve tractor air, and the trailer's spring brakes should set automatically through the emergency relay valve. If the dolly brakes are not holding, the failure may be in the dolly's emergency relay valve or the line between the dolly and rear trailer, so the automatic spring-brake application did not reach those axles. Do not assume the system is safe. Treat the vehicle as having partial or no trailer braking and secure it before moving.
2. What should you do before attempting to move or repair the vehicle?
Read the worked answer
First, keep the vehicle stopped and set the tractor parking brake. Chock the wheels if possible, especially on the downhill grade, because you cannot rely on the trailer spring brakes. Do not crawl under or between the trailers. Shut down the engine and identify the failed line by inspection from outside the vehicle. If you can legally and safely do so, close the shutoff valves at the dolly and rear trailer to isolate the leak, then build air pressure and check that the spring brakes set and release properly. If the leak cannot be isolated or the brakes still do not hold, do not move the combination; call for repair. Before moving, perform a full air brake test, including the tug test and a check that the emergency brakes apply when the supply line is disconnected.