I think I've found part of the answer to my original question.
The antiskid valves should be closed below 8kts wheel speed (system logic). However, the operation of the antiskid module is dependent on more than just electronic signals from the BSCU (Brake System Control Unit) and metered brake pressure (controlled by pedal input).
An active source of hydraulic pressure directly from the #4 system is also required to operate the normal antiskid valves. If this source is absent, the spools in the second stage of the antiskid modules may possibly reposition to allow fluid to escape into the return lines (feeding the Park Brake valve). If the park brake valve is open, then pressure cannot be held.
My initial tests on the aircraft (with no active source of hydraulic pressure) seem to prove this theory. There are no leaks per se, just no proper control of the antiskid valves. The accumulator pressure bleeds off very quickly if the Park Brake Valve is held open (by pulling the circuit breaker prior to parking the brakes)
My next tests will involve an active source of pressure. I'm hoping this will be more effective in holding pressure on the brakes (even though the Park Brake Valve is open).
Cheers.
NSEU