From the Boeing OM - "If the normal and alternate brake systems are not pressurised, parking brake pressure is maintained by the brake accumulator. ............
Sufficient pressure is stored in the accumulator to set and hold the parking brake, but the accumulator is not designed to stop the aircraft."
Seems to me that any situation which requires you to use the parking brake accumulator would, of necessity, be such as to NOT RELY ON THE PARKING BRAKE. Use it initially, yes, but get the chocks in place soonest!!!
