Replace the cap on the expansion coolant tank . If the seal is faulty or old or split it is doing nothing and needs a new one . Plus we have no info on what model you have this will make it hard to help you .You could post a picture of the leak .If it as a leak the coolant wll never circulate. Test thermostat but puting it in a bown of hot water from cold and see if it opens up. Or it might be stuck open .