Why won't the parking brake release on my haul truck?

[Frank Zimmerman]

I'm having trouble getting the parking brake to release on my haul truck. The air pressure gauge is in the green zone, but the release lever won't move. I've tried it in forward, reverse, and neutral, with the foot brake both on and off, but nothing changes. I'm new to haul trucks, and this is my first time operating one. Could I be missing a step, or is there a common cause for a parking brake that won't release?