The parking brake is on the No2 system, with a nitrogen charged accumulator located in the right wing root. According to most SOPs, minimum pressure in the No2 system before engine start is 500psi, anything below that then the system pressure must be restored by operating the manual pump in the right engine nacelle. That is why it is normal to start the No2 engine first on the Q400. The hydraulic pumps are driven of the reduction gearbox, the APU supplies DC power and Air only. The reason most SOPs use 500psi as minimum pressure (or 1000psi if you choose to start the No1 engine first) is because it is possible to have indications of pressure in the No2 system below 500psi but that is most likely residual accumulator gas pressure, and brakes will not function. So it is quite easy to foresee a scenario where an aircraft parked on an incline with little or no accumulator pressure will begin to roll when the chocks are removed or someone cycles the parking brake to test it and unwittingly depleting the remaining pressure.