To add more fuel to the fire, I found a website which goes into quite 
alot of detail about metallurgy and the affect of phosphates on coolant 
systems.  Apparently the owners of British cars have similar 
problems....It makes for interesting reading and confirms what most of 
what the list has been saying; flushing and changing the coolant is the 
best way to prevent corrosion.  

Anyway, the website is at www.team.net/sol/tech/coolant.html

Enjoy!

Scott and Patty
'84 Westy