Looking out a bike that hasn’t been used in a couple of years and the disc brakes barely work/ squeal badly. They have either self contaminated or the pads have self glazed? Resin pads/ shimano brakes. Hoping the callipers haven’t degraded /leaked….. do pads just give up after time?
If it hasn't been used for years, the seals may have dried out and degraded, fluid could be leaking past the piston onto the pads/rotors which would result in sqwarking noise and no bite
The rotors often get rusty. Just sand the rotors and pads and clean them with alcohol, then bed them in again. It might be a good idea to replace the front pads with new ones if you're worried, keep the old pads as spares if they aren't too worn.
One of my old bikes wasn't used for years. Rotors just had a little rust but still worked fine. Didn't even need a bleed. 1998 XT 4 pots and still never needed a bleed. Got it out the garage to clean up to sell (on classifieds) and still work as new.
In order of actions, I would clean the rotors with IPA and sand the pads, then bed them in again and see if it works. If the lever feels soft/bite point not right, then bleed. If all that works for a bit, then the squeal and lack of power comes back, then you need new calipers.
Or somewhere on here, someone has sourced replacement shimano piston seals iirc.
