On my Tektro V brakes I am unable to turn the spring adjustment screw (below the cantilevers, to regulate the position of the pads on the rim) unless I first detach the spring. Rear brakes seem ok but I have this problem with the front brakes. The screw simply will not turn in the hole. Once I release the spring I'm able to turn the screw. I've studied it and can't see what the problem is. Anything I can do to correct this?