My hydraulic MTB brakes squeal after I cleaned my bike My hydraulic MTB brakes stated to squeal after I washed my bike why?
Did you re-lube your chain after you cleaned the bike? If I am not careful when I lube the chain on my mountainbike I get some splatter and if it gets on the brake pads I get loud braking for a while, louder when wet.

Now, I slowly lube my chain while turning cranks backwards, key is slowly, both to get every linkage and not splatter lube. Then I continue to spin backwards slowly for 30-40 seconds, let is set for 5 minutes and use a rag to wipe any excess from the chain so it doesn't splatter once I get going.