As I just posted, (we might have crossed), the hydraulic fluid is NOT trapped; the fluid path is open between the accumulator and the parking brake actuators. Trace the pipework in the diagram I just posted.
"fade out", i.e. pressure reduction, occurs due to micro leaks past the hydraulic seals. This is normal and not an issue when there is a constant supply of pressure from a pump, but if the only pressure supply is from a charged accumulator, that pressure will slowly diminish.