While driving the 310E backhoe on level ground the parking brake came on. I have checked out all of the electrical side. The solenoid activates and I removed the hydraulic hose going to the parking brake while having someone turn the parking brake switch on briefly. I observed fiuld coming out of the hose, but I don't have the proper connections to check the pressure. It does have enough pressure to turn off the parking brake light with the hose connected. I am assuming there is enough pressure to release the brake. There is no excessive hydraulic fluid in the rear axel so I don't feel the brake seals are leaking by. I am at a loss as to where the problem exist. I also disconnected the universal joint shaft to check out if there was a problem with the transmission. The transmission works fine. I tried turning the shaft going into the rear axel with the U joint shaft removed. No luck, it doesn't move in either direction. I am to the point of breaking down the rear axel front end unless someone can offer something I might be missing.
Looking at this parts photo of your park brake. Looks quite simple. # 3 is a spring washer or spring disc, I forgot what they called it but it seems it's the only item that gives the brake the on pressure. So your hydraulic pressure is your release system. So it seems like one of 3 things. 1. not enough pressure. 2. leaking seal, or 3. something broken that causes binding. I think I would try to test the pressure before tearing things apart.