If you are only using water, and have been for some time, then you've caused the problem.The top up should be a mixture of water and coolant as the coolant stops the water from reaching boiling temperature.When you say 'reservoir', do you mean 'expansion tank'?The expansion tank level will be higher when the engine has been running as the hot water expands (hence 'expansion tank'). Once the engine is cool (leave it overnight), the level should be between MIN and MAX.Using only water in the cooling system will lead to high steam pressures that will blow gaskets and introduce water into the oil system, effectively knackering the engine. If you can see a mayonnaise like oily residue on your engine, the gasket has gone and that's a lot of work. Very expensive. Part of the engine could be warped beyond repair.