For the sake of disscusion lets say you buy a 39 year old car that was sitting for 4 years give or take. The cap seems to be a few years old and has that little red lift tab to relieve pressure. You replace the water pump and most hoses and leave the thermostat out for fear it would get clogged with years of gunk. If the cap is bad pressure won't build and the water will leak out. Now lets say this car doesn't have the overflow bottle anymore. If the system leaks water because of a bad cap could enough water be lost to cause a low coolant over heat? Just wondering cause my car over heats and I haven't had time to trouble shoot all problems yet radiator gets rodded this weekend I hope and a new cap will be put on. I also replaced the water pump and bought a new thermostat to put in but left it out when I saw the previous onwer have left it out also. I just wonder if the bad cap may have started a chain of events that led to the overheat.