Accumulator pressure will actually drop over time weakening the parking brake force. Unless the hyd demand pumps are on. There was once when a bunch of guys from my place who were towing an a/c to a remote stand couldn't find any chocks and decided to set the parking brake only and then go for lunch. Coming back after lunch, the airport authorities surrounded the a/c as the a/c actually rolled back blocking an active taxiway. Thank God there were no damages.