Tegger wrote:

The driver insisted the brakes had been set before
he left the truck. I don't know enough about semis to know how this could
have happened.

With great difficulty unless the brakes were completely out of adjustment.
There are two brake circuits, one for the tractor, one for the trailer. When
you pull the button out, you're dumping the air pressure, allowing the
spring brakes to be applied. In other words, it's a failsafe system; lose
air pressure and the spring brakes are applied.

That system can be a major pita in the winter. If the trailer has been
disconnected or the spring brakes are set, they can freeze to the drum. Then
you're laying under a trailer at 5 below beating on the drums with a 5 pound
hammer. Must be the reason I don't drive a truck anymore