Taking the car up to highway operating temperature throughout is preventive maintenance (PM), not a repair procedure. It more thoroughly evaporates moisture buildup than low speed driving can. Do that regularly so you won't have to go out on an open road and do much light braking to remove rust; unless, you just enjoy that sort of thing. Covering wheels is a PM adjunct for rain; but, not humidity unless they're sealed with desiccant inside. Taking the car out for a short highway cruises as necessary will reduce moisture that causes rust buildup so your normal low speed driving and braking will remove what little rust occurs. An ounce of prevention is still worth a pound of cure.

I had a Isuzu pickup that belonged to my brother. It sat for a while and the right front always squaked for a few stops after sitting over night.
Doubtful that you have air in the brake line. Look to your oil change and see if you didn't hook up a vacuum hose which is what Walmart did for me once.

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Knowing these brakes it is most likely rust. I'll be driving it tomorrow so I'll know.