The PIC of this Air Midwest Flight 4408 said that, while on initial climb, power (NP) on the left engine decayed to 60% and would not recover. The engine was feathered by condition lever but the torque rose to 140% while returning for a landing, according to the PIC. The aircraft landed safely. A post-incident investigation disclosed that an oil leak had been traced, the day prior to the incident, to a fractured propeller control unit solenoid. That part was removed and replaced as a precautionary measure. The plastic blade retaining rings inside the propeller assembly were damaged due to the engine overtorque and subsequent shutdown. A teardown of the propeller control unit (PCU) showed that the beta tube had not failed nor was it freewheeling inside the PCU. A spectrographic analysis of oil samples, drawn from four separate sections of the engine, tested satisfactorily. As such, no reason could be found for the drop in engine power (NP) and subsequent overtorque.