I'm as stumped as you. I'm a mechanic but I'd be referring the car to a brake specialist at this point.
Air in the system doesn't behave like that, it would be a spongey pedal every time, not a sinking pedal when held lightly.
You must be so frustrated!
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2001 Boxster S, TipTronic in Seal Grey aka "The Imp"
2001 TE50, ESS in (now) Grigio Titanio Matallizzato aka "The Golden child"
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