How do you decide if rusty hardware is safe to reuse?

Rule of thumb I go by: if the threads are intact and the bolt head isn't loose or cracked, surface rust alone doesn't kill a fastener. The thing to check is pitting. Wire brushing takes off the flaky stuff but leaves you staring at what's underneath. Run a nut down the threads by hand. If it spins clean the whole way and the shank isn't visibly eaten away, the bolt's fine for brackets, shields, and general stuff. If the threads feel soft, gummy, or the nut binds, it's junk no matter how decent it looks on the outside.

Wire brush alone usally isn't enough to judge, though. Soak the keepers in white vinegar overnight, scrub with a toothbrush, then hit them with a light coat of something to keep flash rust from coming back. You'd be surprised how many bolts look terrible and come out fine once the crust is actually off. Anything that comes out of the soak with real divots or scaling gets tossed. Washers and nuts I'd just replace outright. They're pennies each and there's no real way to judge a flaked washer. Same with anything self-locking, those are single use once the nylon insert is compressed. For the mid-level stuff you're asking about, reusing good-thread bolts is completley normal and not asking for trouble. Where I draw the line is anything that was torqued hard or is load-bearing, and you've alredy sorted the caliper hardware, wich is the right call. Also don't clean threads with a wire wheel on a bench grinder, it rounds the edges off the threads and makes them bind. A thread file or chasing by running a nut through is better.

One more thing, keep a coffee can or baggies of the cleaned hardware seperate from the new stuff. When you're reassembling at midnight you don't want to be guessing which pile is which.

One thing I'd add about buying replacements: check the markings on the bolt heads before you toss anything, because those numbers actually tell you the strength grade and a generic hardware store bolt is often softer than what the factory used. I learned that the hard way on a tow hook bracket years ago and had to redo the job a season later. Also take one of your old bolts with you to match the thread pitch, since eyeballing it never works and the counter guy will just shrug when the wrong one strips. The heat shields themselves I'd honestly just replace if they're crusty, they're cheap and they love to trap moisture against whatever they're bolted to.

Did the same split on a bumper and bracket cleanup last winter and the thing that saved me hours was a brass wire cup brush chucked into a drill. Soft enough that it doesn't chew the threads like a steel wheel, and the rust comes off way faster than hand scrubbing. I ran every keeper over one of those after the soak and probably kept a third more hardware than I planned to.

One habit I picked up the hard way: take a photo of each bracket with its bolts next to it before they go in the soak. A pile of identical looking fasteners all cleaned up looks great untill you're back under the truck trying to remember which two were longer.

Update: the drill brush tip was a game changer, ran all the keepers over it and most cleaned up better than I expected. Still stuck on the pitting question though. The nut test works fine on bolts I can reach, but half these bracket bolts are press-fit into the bracket itself, so how do you judge those? Just eyeball the threads?