Will, i have used the copper a couple times with great luck. THink about it for a minute. What whould it "gum up". The only way to get overheating is lack of water, stuck thermostat, bad water pump, or plugged radiator. When you said you have no heat out of the core, i immediately think of low water level. Thats what i had when i had the leak and was using water... air bubble in the heater. The copper seal will plug holes when it sees a differential pressure across a hole. It works on the head gasket, cracked head, and also sees the closed thermostat with the small weep hole as a "leak". It will not plug the radiator or heater core that has a constant flow with no sugnificant differential pressure. I have probalby go 2 years or 30k miles on my F150 since doing the "copper". Heater and radiator work fine...only problem i had was plugging the weep hole in teh thermostat. I left the copper in the radiator for 2 years. It says it will maintain or keep plugging small pin holes that reoccure in the gasket or head. When you flush it out and put in new antifreeze, you can have a leak reoccure that will not be fixed. I cant guarantee what problem you are having, but my success with the copper leads me to believe it is not that.