For me, it was just buy new ones, but I had cheaper calipers.
How did you go between buying new calipers? Hopefully more than a couple of years. Otherwise I think something else might be going on.
Seems almost certain your issue is pad contamination, and if repeatedly changing pads, rotors and calipers hasnât fixed it, seems likely to be coming from outside the braking system.
Do you ride in the wet a lot? Do you transport your bike on the back of a car in the rain with any frequency? How do you apply lube? How and with what do you wash the bike? These would be the first issued Iâd double check.
IME nearly all brakes will squeak a bit when wet, but shouldnât any other time.
Could the circles be caused by the pressure of the pistons on the pads? I did the brakes on my Toyota last year, and the pads had marks from the pistons too. Compression and possible minute piston/pad wear marks is what I thought. shrug
Iâve pretty much given up on a non-honking bike. I treat them and get the honking knocked down a little, and move on.
I did start using a brass wire brush on a drill to âcleanâ the rotors.
And isnât âbakingâ the pads a little overkill? I was strongly urged to not bake my pads under any circumstances by the LBS.
Oh, and careful sanding pads. I found out I ruined a pair of old pads when I saw they were sanded off angle, thinner in the end nearer to the hub. Yikes⌠That was for my fat bike. The frame seems to really amplify the brake honking incredibly well.
I love my galfer rotors and blue brake pads. I switched from SRAM because my rotors wouldnât stop warping and squealing. The Galfer rotor is also surprisingly lighter.
What pad material are you using? Iâve been running SRAM organic pads and itâs been dead silent even in the rain. Have had to sand em a few times already due to rain but still theyâre quiet.
Stop cleaning your bike, seriously, it is the guaranteed way to contaminate the pads and rotors.
Iâm running Avid BB5s with 160mm centrelock shimano SM-RT64 budget rotors and sintered pads, the only time I ever have brake squeal is the first time I put the brakes on after they get wet( from riding in the rain), the key is to keep pulling hard until the squealing stops, after that first application silence.
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Not had brake squeal for a while now. Or maybe only occasinally when itâs wet or maybe a bit of contamination went on it, but it goes away quickly. I donât do anything special to keep them clean, just wash with soapy water (dishwashing liquid) along with the rest of the bike after basically every ride, unless itâs dry.
Consistent squaling more likely comes from the bike, then the type of pads and rotors you use. Either the caliper mounts are not flat and need facing, or maybe the fork isnât stiff enough for disc brakes (had that with a QR bike).
If you think the calipers are leaking, you could replace the piston seals, or replace the whole calipers.
Itâs strange how my MTB disc brakes have been so quiet over the years, but trying to keep the brakes on a road bike quiet is another story!
If you havenât already, I would recommend Uber Bike Race Matrix pads. I run them on all my Mountain Bikes and my winter road bike, never have any problems with brake squeal, my summer bike has Absolute black pads and they squeal any time they are even slightly wet.
Based on everything youâve tried and the fact youâve replaced calipers more than once Iâd echo @Gnome and suggest checking whether the mounts need to be faced. I can imagine if the facing is bad enough it could even make leaky seals more likely (if theyâre forcing the piston to sit wonky in the caliper) but I suspect it would have to be unusably bad before that would happen.
Also worth taking a page from @batwood14âs anecdote and doing a really thorough investigation to rule out some really weird unexpected contamination source like a third party with cleaning spray when youâre not even home.
And Iâll echo a few different warnings about being too obsessive/proactive in trying to address issues too, but not because of additional contamination risk as suggested by others: The lesson that took me longest to learn is that all of the âfixesâ really only get you about 90% of the way there. The only way Iâve ever found to truly finish the job is just to ride hard for a couple hours on a really dry day, and that alone is also enough to deal with quite a lot.
Hereâs how I phrased it in a different thread (shamelessly including the endorsement I got from another user 
)
And another version of much the same info that I wrote out for someone in my clubâs facebook group:
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Size/weight of the rotors I think. Itâs the resonance of them and it seems that the normal sized rotors on a road bike and what theyâre attached to is perfect for creating an ungodly racket.
Iâve got an urge to try 180mm Magura 2mm rotor on the front of the adventure rig to see what thatâs like.
Cheaper rotors might also be quieter since theyâre generally heavier than top end ones.
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Random thoughtâŚthereâs no way whatever youâre cleaning the bike has polish in it? I know someone who used âwash and waxâ designed for a car and had similar problems with âmysteriouslyâ reoccurring squealing until he stopped using it.
Galfer. Light. Strong. Quiet.
Ambient temperatures donât make enough difference to brake operating temperatures to bother thinking about. I live in Aberdeenshire and have no problem running sintered pads on all my bikes. Descending off the Cairn-O-Mount at 80km/h and braking hard at the bottom to make the sharp turn certainly gets my brakes warm enough whether its 0C or 20C.
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Where do you ride, and what are the specific conditions?
My MTB is pretty quiet too. The worst bike is the fat bike (2016). Shimano Deore XT, and I went through dozens of pad sets and new rotors. I would get it silent, and then on a ride it would start honking. I went from an aluminum frame to carbon which helped attenuate the noise quite a bit, but they just squeal/honk like itâs a feature, not an aberration. The Roubaix and Aspero go from fits of relative quiet to obnoxious amounts of noise, and I just got used to it. The drama and work to get to total quietness just isnât worth it to me. The Aspero does squeal if I really brake hard. I just donât brake hard as much. It has GRX 400 calipers and Iâd like to get everything on it to 800, so maybe that will help, or not.
I did laugh once, out on the fattie: I hit the brakes and the incredible volume of the honking scared a bunch of napping deer. 6 or so of them jumped up looking my way, and took off like a shot. I might have scared them, but that many deer appearing to levitate and take off shocked the heck out of me for sure. Wow! I did feel a little guilty for waking them, but they are deerâŚ
Are you bedding new pads in properly?
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I was watching a car rebuild show on TV - the mechanic was busy changing the brake pads and metioned that he checks for a chamfer on the leading and trailing edge of the new brake pads. according to him without a chamfer the brakes scream, so when they come from the OEM without the chamfer he takes a smooth file and files them himslef which eliminates the brakes squeel.
Havnt tried it myself yet but maybe itâs a solution to try
This is an interesting thread, I just dealt with this on my Sonâs Epic. I found that his front XT 2-piston had some minor leak inside the caliper. I pulled it apart and cleaned the caliper and it has been dry ever since.
I moved him to the same setup I always run with my XTR Race. Shimano Ti/Metal pads with Ashima AI2 rotors. Here in the midwest, a very quiet combo.
I think this might be a bedding issue. Make sure you bed the pads properly before riding them brand new.
This might be a good read. STOP! How to get more power from your MTB disc brakes | ENDURO Mountainbike Magazine
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Lots of good advice here, but Iâll add a couple things:
Contaminated pads are done. Pitch them. Rotors can be cleaned and resurfaced, but oils absorb into the pad, and even if you clean and resurface the pad surface, they eventually start leaching out again.
Whatever you do, keep oils away from pads and rotors. Donât touch them with bare hands, make sure brakes arenât leaking. And another reason why I like wax instead of wet lube. If youâre going anywhere near pads and rotors, it helps to keep a box of latex gloves around your work area, plenty of paper towels, rubbing alcohol for cleanup.
Bed your pads properly.
I still have some sintered / metallic pads for my MTB that will squeal like hell - thatâs just a design issue. But, plenty of pads that donât.