if the controller doesnt see an engine speed signal, the brake will not unlock, also if the signal is intermittent the brake will unlock/lock.
1st thing i would try is remove, clean and reinstall the speed sensor.
sometimes they just get dirty, or it could be damaged.
when you reinstall the sensor, back the jam nut off, screw the sensor in by hand until it contacts the flywheel, then back it off 1/8 or 1/4 turn and snug the jam nut back down.
disconnecting hoses from the brake valve and plumbing in a porta-power to both of them is the only way i know to release the brakes. it's still not gonna roll easily.
i have a quick check to "isolate" the speed sensor, but it involves a $1000 bobcat remote start box, and bobcat service analyzer