Checking Air Lines and Electrical Cords Between Units
This domain tests the connections that carry air and electricity across each gap in a double or triple. Every added unit multiplies the number of couplers, and a single leak or unplugged cord disables a whole function.
Air lines
Each connection carries two lines: the service line (blue) that signals brake application, and the emergency line (red) that supplies air and triggers the trailer spring brakes if supply is lost. Glad hands have rubber seals and must seat squarely, red to red and blue to blue. Shut-off valves stay open on lines in use and closed on unused couplers to keep out dirt and hold pressure.
Electrical cords
The seven-way cord carries ground, tail and marker, left turn, right turn, stop, and a charge circuit; the ground wire is white, and other colors follow the vehicle's diagram - verify rather than assume. Secure cords with ties so they neither drag nor pull tight on turns, and leave enough slack for offtracking.
Leak checks
After connecting, listen at each coupler and watch the gauge. With the engine off and brakes released, a combination vehicle should lose no more than about 3 psi per minute; a faster drop means a leak you must find before moving.
Worked example
Pressure falls from 120 to 95 psi in ten minutes after you add the second trailer - about 2.5 psi per minute. That passes the 3 psi rule, so the drop is normal system bleed, not a defect. Swap in a coupler with a torn seal and the same test will show a rapid fall.
Traps that cost marks
Crossing red and blue; leaving a shut-off valve closed; cords routed over sharp edges or dragging; no slack for turns; connecting the first trailer and forgetting the second; not rechecking after a tight turn loosens a coupler.