I see. Pressurisation to that extreme extent suggests a duff sensor in the pump. Regular pump activation along with FW leaks is the common problem – obviously the leaks cause the sensor to call for more pressure. But extreme pressure in the tank must be a duff sensor. When I had mine our for a strip-down I noted the model number and a new replacement is still available at a not too excessive price. Given the original’s age and the severity of the problem you might save yourself hassle by simply replacing the pump.