Firstly I'd like to mention that I don't think this issue is related to contamination (at least from sprays etc) and I'm aware of the following 2 threads:
I'm having trouble with the rear brake on my mountain bike. It's running Shimano XT (BR-M8000). The braking power seems very feeble and I'm getting a squeal as the bike slows to near-stop.
So far I have tried cleaning throughly with alcohol, brake cleaner and even taken the rotors of and cleaned them with soapy water followed by isopropyl alcohol. I've keyed the pad with emery paper and even swapped the pads out.
I've bled the brake twice and the initial feeling through the lever is good. I did my best to get all the air out and extended the reach to get a greater pull on it.
I wondered whether I'd glazed the rotors and replaced them with brand new (and cleaned) rotors as I'd been meaning to do so for some time to reduce noise.
My new rotors a some Shimano Ice Tech 160mm centre-lock so should be the right gauge. I've bedded them in without dousing with water as some do. The front brake is a larger rotor but bites really well. I can't even skid the back wheel.
I don't think the calliper is leaking but I haven't done extensive checks to see. I did get what felt like a much better brake when I got them covered in sandy mud. After I cleaned the bike with muc-off the brake was back to being pants again.
My next steps were going to be to check the brake hose for weak spots and/or bulges. Replace the hose if needed.
Last option is to empty the system and clean the calliper and remove, clean and reseal the calliper pistons.
Is there anything else I can do before I go down these routes?