Alapimba,
The water in the "tank" is not any fresher than the rest of the system. All of the water recirculates through the entire system, whether it has one, two or twenty radiators.
If rust is a real problem, then you can add some rust inhibitor. However the items used in a water cooling system are generally made of materials that stand up to water very well (aluminum, copper, cast iron), particularly if the system is sealed and oxygen is generall excluded from the system.
The reason to increase the release pressure of the cap is to increase the overall pressure that the radiator system can hold. This will raise the boiling point of the cooling water. In may race car I use pure water, with a water surface tension release agent (no alcohol as the rules do not permit it). I use a 24 lb cap and even at 220 degrees F (104 C) I do not lose water.
I am quite confident that if the water system is properly plumbed, can hold pressure of at least 15 PSI that the standard Fiat water pump is quite capable of circulating the water in the system.
A electric fan has the benefit that it can move the same amount of air whether you are moving or standing still. This makes the assumption that it has a good shroud around it that is properly sealed against the radiator core, the core is clean and air can be pushed/sucked through it.
Doing analysis of cooling systems by remote control is always a problem. All I can do is tell you the physics that come into play in ANY cooling system.
Regards,
Paul
