So years ago the Willey's Jeep had a drum for the emergency brake! They would loose holding power after a while. The engine wouldn't hold it on a good grade. Yes sometimes the owner would return to find his Jeep in the bottom of the reveen!
Anyhow Chevy powered there parking brakes with electric hydrolic pump to maintain pressure on the brake.
If the pump runs intermittently,then the switch on the linkage, likely needs adjustment or replacement.
No guarantee, but that's where I would start.