Couple of weeks ago, uncoupled my M818 from my M871 hay hauler trailer and discovered that the truck brakes didn't work well - evidently, the trailer air brakes were doing most of the braking work. Found and fixed a leaking fitting that had drained out the brake fluid, and replaced the DOT5 fluid and bled the brakes - all is working well again. I needed the M818 to travel bobtail to Ft Polk to pick up a M447C trailer, so was glad the brake problem was a quick fix. 350 mile round trip, and the M818 engine purred along for over 12 hours straight with no hickups.
Then used my M923 to flat tow a non-running M925. I made up a towing light cable out of two trailer plugs and a commercial 4-wire trailer wire setup. Plugged into the M925's trailer socket, but had to unplug the turn signal switch so the turn signals worked. Used bullet connectors, so the light setup wasn't permanent.
I unhitched the M447C trailer, then hooked the M871 trailer back up to the M818 and made my bi-weekly 40 mile round trip run for horse hay.
I like the M939 series auto transmission for traveling in traffic, and the extra cab room, but I still like gear jamming the M818 manual transmission.