If the engines are shut down, hydraulic systems 1 and 2 will not be pressurised, and therefore, the remaining pressure in the system will slowly deplete over time, as well as with any demand that is applied to the systems. 

With both hydraulic systems inoperative, hydraulic accumulators in the alternate brake system will provide hydraulic pressure for the brakes that is sufficient for a limited number of brake applications. If the accumulator pressure in the alternate brake system decreases below a threshold, there will be insufficient pressure to operate the brakes. If the aircraft has electrical power, it is possible to increase hydraulic pressure in the alternate brake system by temporarily switching on one of the electrically driven hydraulic pumps. The controls for the electrically driven pumps are located on the Hydraulics section of the overhead panel. 

The alternate brake system pressure indicator is located on the main instrument panel, below the gear lever.