Repeated Disk Contamination I have a set of Shimano m785 calipers with Saint RT81 rotors.
Every week or so they squeal and severely lose braking performance (mainly the front).

Using disk brake cleaner and a touch of sanding I can get them working again. When cleaning there is clear contamination from oil that has dried.

Now I've thoroughly cleaned the caliper and even simultaneously replaced the pads and rotors. Two weeks later and it's back again. I do ride past some moderately industrial areas but where is the oil coming from? I don't think there's a leak from the caliper as the lever doesn't feel spongy.

Is there a way to prevent this other than catching the oil early each week and removing it?
It's probably some kind of unfixable (given the lack of replacement parts) problem with the pistons. If you want you can take apart the caliper to confirm, using a blower attachment on an air compressor to pop the pistons out and inspect them. But what it comes down to is if your no-service-parts-availability caliper is leaking oil repeatedly, there's not a lot you can do but replace it. Luckily a new BR-M785 complete with finned pads is under $50US.