I recall the '74 parking brake system as follows...please correct if my memory fails me...
The individual anti-skid valves are closed until commanded open to release some small amount of pressure to get the skidding wheel to rotate again...
This small amount of pressure results in a small amount of hyd fluid being released back to reservoir, tracking via the park-brake valve (which would at this point be open).
At the gate (or anywhere else), when the park brake is set, the rudder pedals (one or both) are depressed and mechanically held there by selection of the park brake.
That allows hyd pressure (with accumulator pressure when the system is not pump pressurized) to be directed to the brakes selected by, one or both rudder pedals that are depressed.
The pressure is trapped between the park brake valve and the brake units to keep the brake applied.
If you release either the pedals (the pressure supply valves), or the pressure return (the park brake valve), the pressure between those two places is released, i.e. the brakes are released.
The problem here is, when you release the park brake to, say, enter and line up, and the rudder pedals release but the park brake valve does not (i.e. it stays closed), the park brake pressure will release and you will roll for the line-up to takeoff....BUT
..with the park brake valve still closed, any operation of the anti-skid valves to release any skidding wheel will have nowhere to go, that is, the anti-skid system will not work, and there is a flight deck indication when this malfunction occurs.
If it does occur, simply cycling the park brake will fix in most cases.
If it does occur, no real problem for takeoff (or landing), just keep in mind that you will have no anti-skid system available, and remind yourself how much runway in excess of that you really need, is available.
EW73