I've just picked up a pair of Tektro 510 dual caliper brakes to put on a bike I'm building up from used parts. Nothing fancy. The calipers have the original brake pads (pretty worn down), with integral bolts. Two of the bolts are stuck fast. The hex key will not shift them and in pushing the key with so much force to try to undo the (soft metal) bolts, I've rounded off the hex bolt head. Any suggestions as to how to get the bolts out, please?