hi i need some help with mechanical disk brakes. i have a norco katmandu with just basic disk brakes however after about a month and a half my back brake no longer engages. When I pull the brakes all i hear is a grinding rubbing sound and it barely slows me down. The brakes are pulled all the way until the literally touch the bars but nothing happens I need some help and I am hoping to be able to fix it without having to bring it into a bike shop. any suggestions are greatly appreciated thanks.