Fuel Venting from left reserve tank

[GMPoland]

Seems my normally reliable skymaster has developed a leak that has me puzzled. After the last top up the FBO called to inform me that there was fuel leaking from my acft. closer inspection revealed that the left inboard was overfull and leaking from the vent. left main that had been topped of was down slightly. Draining from the reserve stoped the problem only slightly. Next day same thing. I drained the reserve and aprox 15 Gal from the mains and the leak has stoped. this is concerning me greatly as the fuel tank selectors were in the off position during all this. I have the parts and service manuals and I can only think that I have one or both fuel selector valves that have gone bad. My acft is a 337A SN 0127. I drained this fuel from the front and rear fuel gascolators. I dont think this this should be possible with the fuel valves in the off position. My local shop is great but has little experience with the Acft type. Any comment on this?