The accumulator is charged with full system pressure - of around 3000psi, (A330). This pressure pushes on the free dividing piston in the middle of the accumulator cylinder. This piston is free to move and holds nitrogen gas on one side and system hydraulic fluid on the other. So when the pumps are running, the hydraulic fluid pressurises the nitrogen gas to ~ 3000 psi.
When the hydraulic pumps stop, the nitrogen in the accumulator presses on the free piston to maintain ~ 3000 psi in the hydraulic fluid. When the parking brake is set on, this nitrogen gas 'spring' in the accumulator keeps the fluid in the park brake circuit pressurised.
If the brakes are repeatedly applied and released with the pumps off; the free piston in the brake accumulator will move a little each time until eventually it reaches the end of its travel inside the accumulator cylinder. At this point the accumulator cannot maintain the hydraulic system pressure. On Airbus, the brake accumulators can apply full brakes about 7 times before they become exhausted.