A leaky core is caused by several factors... Aluminum is more vulnerable to electrolytic corrosion than copper or brass because aluminum is a highly reactive metal. When the corrosion inhibitors are used up and the pH of the coolant drops to 7 or below, aluminum becomes a sacrificial anode and is eaten away.
This same type of corrosion can also occur even when the coolant is in good condition if the engine does not have a good ground connection. Voltage from the charging system will flow through the coolant to ground, creating electrolysis corrosion that attacks the components in the cooling system. Checking your ground wire is a good thing especially in an older MN12.
I am of the opinion that the stock grounding wiring is insufficient and should be upgraded to a higher gage wire size. My opinion is strengthened by Ford CYA TSB 01-15-6
. Or cars are getting old and the wire corrodes. I replaced my engine ground with a Zero gauge welding wire and have no heater core issues...
Checking the pH of the coolant with chemically-treated test strips can help you determine if the coolant is overdue for a change. The alkalinity of a typical antifreeze/water mixture will vary depending on the additives in the antifreeze and the ratio of ingredients, but is usually somewhere between 8 and 11. Higher is not always better, because some of the new long-life coolants have a pH of only 8.3.
Here are my recommendations:
1.Use deionized or distilled water for your radiator. Never use tap water (chlorine ions Cl- can destroy aluminum).
2.Check your ph (see my comments above)
3.Use antifreeze with an anti corrosive inhibitor.
4.Upgrade your engine ground wire. The TSB above has instructions on how to test grounding.
5. Since you are there I would install a 1 way ground wire on the core. You can get them Here
!. Do not use a normal ground here as it can draw current into the core.