I just had my NEW Hall radiator repaired for leaks, and all the joints re-soldered, which cost about the same as a new one! Anyway, before I pulled it I had to break the cooling lines for various reasons over 1,500 miles or so. Each time, brown coolant. I was told by the re-builder that's what comes from cast iron heads, and not much you can do about it except maybe use a silicon based coolant, and service the system more frequently. I thought the engine passages were sleeved, or there was some built in protection from this. The rebuilt stock engine has about 3K miles on it.
Does this sound familiar? Does the type of coolant make much difference?