As the truck rolls along level ground, the driver would occasionally have to run the engine to keep the air brakes pressurized. If the pressure was too low, the brakes would switch on automatically, bringing us to a stop.
On very rare occasions, the engine would have to be run while we were going uphill, making the pushers mad because anyone who saw this would think we were driving, not pushing. But the truck was NEVER DRIVEN AN INCH, we would all have felt like cheats if we had.