2001 chevy c6500 electric parking brake issue
i work for a towing company and been having issues with electric parking brake. it has a yellow push knob that locks the rear brakes. it has a drum right after transmission going into drive shaft. there is a black box by the brake pedal that tells me when the brakes are locked and unlock. 0 is fully locked and 99 is fully released. when parking brake is release it will read 99 and randomly fall down to 90 or 80 while driving. i will have to pull over and set and unset parking brake to fix the issue or sometimes it will go back to 99 on its own. any thoughts what the problem could be?