Check the park brake solenoid wiring harness adapter... I'm guessing you have a newer era truck. Solenoid has 4 wires vs 2 wires for the old era. New the solenoid head points towards the mast and is black in color, old era is screwed into the valve body of the parking brake valve body is silver and points to the radiator. I just had the same issue happen with a F88 code. I used an ohm meter and checked for a short to ground within the wiring harness, and determined the issue was the wiring not the parking brake. I also checked the pump pressure and changed the hydraulic oil and filters which made a bit of difference. Also check the suction line to the brake pump.. if it gets air around the clamps you will get weird readings.