85.5 240 Turbo, AW71...

After a couple years, I've returned to 'conventional' cars. (See sig)
Either I'm rusty, or plain missing the obvious. I could throw parts at it, but the money isn't there.

The issue:

The car intermittently suffers from a brake pedal going down about half way before braking force is applied. Pedal is marginally soft. This usually happens after a good long drive coming off the freeway (spooky) but sometimes arises straight from start-up. Idling and re-starting sometimes makes it go away.

When it's not doing it, the pedal actuation and force is perfectly normal, or better (though not firm like a no boost situation).

I never get a "brake failure" indicator, and the switch tests ok.

Thus far I've eliminated the booster and check-valve, rotors & pads. Hoses are older, but there are no visible leaks anywhere in the system. The master seems good until the symptom arises.

Any leads?

Thanks in advance.
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71 ID21F - 88 CX GTi - 84 244Ti