Erm, are we sure about that? My understanding of hydraulic brake systems is that the park brake pressure is locked in. IE even if the accumulater pressure were to drop, the locked in park brake presuure would remain.
For the OP
The B787 has electric brakes. The brake packs are pressed together by rotary electric actuators. When park brakes are selected they adjust themselves over time as the brake temperature drops, applying a continuous force.
Hope this helps.