Something I found awhile ago, but have not seen mentioned on the board. Fresh coolant has rust inhibitors and and other additives to make components last longer. We all know that over time, all coolants lose the effectiveness of those additives and should be changed. Anyone who's ever taken apart an engine or cooling system component will find metal surfaces discolored - generally a tint of the shade of the coolant used. The tint is due to leaching of the rust inhibitors into the metal. That's a good thing. When a component is replaced, that new component will rapidly absorb some of the inhibitors carried by the coolant. If you have 'old' coolant, it could effectively reduce the inhibitors to a level that does not give adequate protection. Therefore, it is recommended that with any component replacement (pump, head rebuild, radiator, etc) new coolant should be used. YMMV