A320 Cat 3 dual inop
 

Just curious to know why when the parking brake is set to on and the brake pedals are pressed temporarily CAT 3 DUAL is inop on the status page.
My understanding is that when the parking brake is applied then the brake pedals don't do anything I.e they are disconnected.
Would this be a spurious signal sent from the BSCU to the FWC?

Failure of anti skid causes down grading to CAT III single. Setting parking brake disconnects normal braking, may be no signal from anti skid causes this.

Any idea why it would happen only when you press the brake pedals with the parking brake on?

Having thought about this. The only braking failures that cause CAT 3 DUAL inop is NWS or anti skid fault.
Must be the BSCU detecting a false failure of one of those temporarily when you press the pedals.

Pedals pressed but no G hyd pressure applied (due Y prk brk priority) -> incorrect state detected? Cat3 dual also not available during G hyd lo pressure.

Just a speculation...

Another theory from looking at the AMM. When you press the brakes and the braking pressure is more than that applied by the alternate system for parking brake then the normal system will apply braking to meet the demand from the rudder pedals.
Maybe it's from pressing the brakes and the BSCU realises you need some pressure from the green system but of course it's not available.
So it detects the normal braking system is inop. CAT 3 Single.
Clearly the FWC has inhibited showing whatever causes the CAT 3 Single.