I'm guessing you have an F series machine with BOSS. A little more info on what machine you have would be handy. First off assuming this is a F series, look at the box behind your right shoulder. That is the BICS unit. There are five LEDs on unit. All five need to be light for everything to work. Does your boom work? Let's say it does, with the seat bar and button pushed, what does the traction light do? If there is a fault, it will blink a code. If it stays out, I have a thought. Since you are getting a hydraulic code out of the blue that will not unlock brake function. I think you have an rpm sensor problem. Normal F series do not have an rpm sensor, but BOSS does. If no rpm signal, it does not think the engine is running. BOSS sets a code because it sees something it thinks it shouldn't and the brake won't release because it wouldn't if the engine isn't running. Pulling the sensor out and jamming a dowel in the hole to polish the flywheel with the engine running may solve your problem. Other wise the sensor could be at fault. First thing first, check for BICS blink code, very well could be a bad brake solenoid.