Y'all over looked something, the bike has been siting up from what I read, I bet the piston is sticking and then heat is building up in the caliper from the dragging pads and putting pressure on everything. But I would check what they have alreay stayed first because they are easier to fix but if those do not work.
take the caliper off the rotor, take the pads out of it, press the break until it popes the piston out, have something under it to catch the fluid. I bet you will find some nasty looking sludge in there. just clean it out and clean the piston off really well. be careful to not hurt the O-rings in the caliper, once it is all clean get everything wet with fresh break fluid and carefully slide the piston back in with the bleeder open.be careful not to let it get tipped as you slide it back in it needs to stay strait to not cut the O-rings.
it's a easy job to do just be slow and careful doing it.
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"My mind jumped off the high dive into the deep end of the gutter and took scuba gear with it" Me.