747-400 Park Brake
Leafing through my training notes on the 747-400 Park Brake, I started to wonder why the Park Brake Valve is really required.
The notes tell me that the park brake valve is situated in the hydraulic return line attached to the Normal Antiskid Control Module/s. When antiskid is activated during a skid, the brake fluid is ported away from the brakes through antiskid valves, through the hydraulic return line and then through a normally open park brake valve, back to the hydraulic system. At the gate however (wheelspeed less than 8kts), as far as I can see, the antiskid control module will keep the fluid return ports closed and all the pressure coming from the antiskid module will be applied to the brakes.
Basically, is the Park Brake Valve only there as a backup, or have I overlooked something? Maybe the antiskid valves are, by design, slightly leaky?
Thanks.
Cheers.
NSEU