Performed a heating pm on an oil boiler. Brushed and vacuumed. Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. System is operating properly. The pressure relief valve has a slow drip. Found the extrol tank is going bad and water has leaked past the diaphragm into the air side of the tank. Recommend replacing both the extrol tank and the pressure relief valve. There is some corrosion around the domestic coil gasket from a previous leak. Recommend replacing the gasket. Isolated the boiler and drained the pressure from the system. Replaced the extrol tank and pressure relief valve. Filled and checked for leaks. Opened the valves and started the system.