Park/Trailer valves

[Chamberpains]

Nope. They're designed to do that. They don't want you setting one brake without the other.

It's a design that was set in play a long time ago to keep dumbass drivers from setting there tractor brake and not the trailer then hopping out and watching the whole unit roll down a hill away from them. Because the tractor brakes alone weren't enough to hold the whole unit back.

And also if one unit loses air rapidly the other one will lock up to begin slowing the unit down and avoiding a no brake runaway situation. You take your pick on which thought was in mind when they designed it.