Cat D300E parking brake question

5KS00496 One of my clients is "borrowing" this unit for a series of jobs on a coffee plantation next to where it lives. Part of the borrowing is that I need to fix all of the things that come up on it while they are using it. It is old, abused and poorly maintained, so I have gotten to spend way too much time with it over the last few months. I have posted a few threads about it because it is a bit out of my wheelhouse. Too small for me to.know anything about it from my time in mines on the mainland and too big for any of my island clients to have one. The forum has been super helpful, especially Nige. I found out the hard way that the parking brake doesn't work last week. I had to pull the right front wheel to reseal one of the brake calipers. I had the rear wheels chocked and was on what I thought was level ground. Walked over to my service truck to grab something and heard the machine hit the ground. It rolled over my wheel chocks and jack stand. Luckily, there were a couple of machines nearby to get it back up. So I blocked it up under the front articulating frame and finished the job. But I have been wondering about how the park brake circuit works on this rig. Most likely, the park brake hasn't worked since way before my client put it into service and none of the operators thought to mention it. Anyway, I haven't a clue how the park brake is supposed to work on this. Is there some sort of driveline brake? a valve to trap service brake pressure? Or maybe something inside the transmission? Any ideas are appreciated. The only help I can offer is a pic of what I assume is the park brake switch.