I am not familiar with the 747-400 but with the 757. The Flight Deck brake accumulator pressure indication is typicaly sensed at the Hydraulic output of the brake accumulator,the brake acc precharge pressure by a mechanical direct reading gauge sensing the N2 pressure.The low inflight pressure in the 747-400 picture is probably a combination of a lower than normal N2 precharge pressure and a long inflight cold soak reducing the N2 precharge volume . From experience with the A300 and 757 I can positively say that a low acc N2 precharge pressure will reduce the number of effective emergency or parking brake applications available. This was the usual line engineers warning signs of a low N2 precharge pressure. With the brake accumulator precharge correct,but a hydraulicaly empty acc the flight deck gauge will read zero but the acculator precharge gauge (usually in the gear bay or wing/body fairing) will read N2 precharge pressure.
The 757 precharge pressure was reasonably critical but the A300 less so due to the different accumulator design and increased volumetric capacity.