there is a way to release the brakes manually but then you wouldn't have any brakes at all, the manual release is for towing the unit only.
it sounds like either the park brake switch is not working or there is some issue with the brake valve. This brake units are inside the front drive axle, there are two brake units that are hydraulic controlled. The brake valve is inside the dash area above the brake pedal assy. It is not servicable, if there is any problem with it it has to be replaced.
You REALLY should call a JLG service tech on this one.