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I sold a heater core to a CF buddy and tested it today before shipping it out. As basic as it sounds maybe someone wasn't aware to test all new cores for leaks. I just run 20 psi into one end and plug the other while the whole thing is in a bucket of water. This one was fine and it was also brand new, never used.
If you don't have a compressor, you can do the same thing with a bicycle pump and inner tube. I tested mine by purchasing the smallestinner bicycle tube I could get (27" tire I guess) and cutting the tube off 5" on each side of the valve.
Build up the heater core tubes with some electrical tape and then clamp the tube on using nylon cable ties.
Submerged the whole thing in the utility sink for 24 hours.
Wish I could take credit, but I got the idea on the forum.