The so called direct action WC cisterns are thought to be much more reliable than the older types of cistern. They can be used in a high level, or a low level and close coupled styles. These cisterns are usually made from either ceramic or plastic.
The warning sign for an airlocked radiator is, either a cold lower part or cold upper part of the radiator. This might be a symptom of another problem, so have a check of all the radiators and make sure that there are no leaks etc.