Our car has had an interesting issue for awhile. If you hold the brake firmly while stopped for around 30 seconds, the pedal suddenly sinks in about an inch. The sinking is accompanied by a sound that is sort of like exhaling. The brakes don't appear to let go, and the pedal feels firm until it sinks in (and then it is firm again). The pedal has never sunken except when stopped.
I haven't bled the brakes yet because this is not the normal spongy feel you get when there is air in the line and I've been too lazy to investigate this. Braking performance is good under all conditions.
Is what I have most likely benign, or is there some potential for there to be a real problem? Part of the reason I haven't worried about it is that I've noticed the exact same behavior on some of the fleet cars I've driven.