My coolant resivoir reads max when cold.... drive it till temp gauge is at 12 o'clock and sooner or later the low coolant light comes on . I check the resivoir and coolant is down at min - which triggered the sensor. If I slowly let the pressure out of the tank the coolant rises back to max.. Is there so much pressure in my system that it forces the fluid down ? What is causing it? Thanks for your ideas.