Our 770G has no park brake what so ever. Wondering where the park brake valve is, If it s right on the front of the axle.. wanted to check pressure and signal from the controller but I cant remember where the valve is. Al-tho if it wont hold its prob shot as it takes hyd pressure to release if I remember right.
I can't help with exact location. But I had a conversation with my dealer tech severals years ago and he said pressure releases, no pressure activates. Then you don't have to worry about it rolling away if pressure bleeds off.
Serial number would be a big help. Does the park brake hold with the engine off? If it does then you may have a bad solenoid. If it does not, then you have a park brake issue. Park brake is integral to the transmission and is mounted on the transmission output shaft. It is spring applied and hydraulically released. The solenoid for the park brake should be mounted near the park brake.