The original post posed the possibility of towing without interconnecting the brakes. That's where the discussion progressed from.
I don't remember what the speed limit is for towing that load with out interconnecting the brakes but it should be in the M809 truck -10 and it was low, maybe 15 MPH?
I'm not very familiar with the trailer brake system on an M818 but I think it has a tractor protection valve to protect the tractor from a sudden air pressure loss. If not, I'd think one could be added to any truck? Maybe someone with more experience could chime in.
Regarding towing a sprag equipped truck and backing up; the TMs for the tow bars state in the safety warnings to NEVER back up with a tow bar. That's due to the pipe sections being designed to take a load in tension, not compression. They can bend or break very easily.
Another issue is pintle, shackle point and tow bar strength. The towing limits also reflect the ability of the tow bar, bumper shackles, frame, frame extension (if any) pintle and pintle mounts etc to handle a particular load. Above the "no brakes" rated loads, speeds etc. there is a stated requirement for a braked load. The equipment simply is not safe or legal to operate over those limits.
I'm not sure whether "they way I've done it" or "what we did in theater" are the point. Tactical necessity can cause you to do some pretty wild things. I've recovered M1A1 tanks with HEMTTs when no M88s were available. It's a 5 MPH job but can be done.
I think that we have an obligation to discuss safe, legal ways of doing things and avoid or take-off any site discussion of illegal and unsafe practices.
Lance