good to know that your problem resolved...
SAD to see the condition of old one...this item is meant to last till the life of vehicle...
to me the cause of this is using tap water in cooling system...the coolant circulation pipes are badly corroded...
my advice is also do consider the situation of other parts of cooling system and make necessary repairs and use a proper coolant from now on...
everything shud be according to factory specs, dont listen to any mechanic etc, cause you are way too much learned as compared to them, as for the cooling system,
1. thermostat valve is must.
2. fan must be in automatic function
3. the radiator cap shud be according to spec and spring/seals shud be inspected
4. no leaks in the expansion bottle and pipes leading towards it..
5. use of coolant and air shud be removed from system after replacement.
6. try to rinse the system to remove any dust particles etc..
if the system is working properly...then after the engine has obtained optimal temperature...and fans have been kicked in a couple of times and just stopped...there shud not be any un-necessary pressure built up in system...