Thanks for your reply Rick. You are correct it wasn't the pressure differential switch. Apparently the previous owner had flipped the lines on the master cylinder and there was some internal proportioning valve/check valve within the m/c not allowing the line lock to release pressure.
The large reservoir is for front disc brakes, the small reservoir is for rear drum brakes typically. He had them the opposite way. We swapped the lines at the master cylinder, bled the brakes and it works like a charm now. Will probably have a new biondo racing line lock for sale here soon.
Thanks!