Most of the time it is caused by fluid or grease getting on the linings. Like Smith says, it may not be enough to leak out on the wheel or backing plate.
Pull the front drums off & see what you got. File or sand a bevel on the edge of the linings. The reliners used to do this on new linings all the time but they don't put much attention to drum brakes anymore. The sharp edge can cause inconsistant braking.
George
