Re: at wit's end
What type of brake calipers are you using? It could be the calipers or V brake arms chattering causing the squeel... I would look at the brake arms and either replace with something more rigid or get creative with some RTV in places it can't be seen to help dampen out that vibration or resonance that's causing the squeal. Also, if it's a V brake setup you might just need to swap out the plastic bushings and those teflon washers etc...