I can only guess. Sounds like either too much pressure or some acid got in there. Epoxy Putty and JB Weld have been long-time friends of mine! I once replaced a heater core in a car, only to nick the new one putting in a better radio. Rather than replace the new one (changing ANYTHING in a 1981 Chevy Citation was hellish experience--my mechanic had nightmares about the carburetor!), I used some epoxy ribbon putty, and never had another problem...with the heater core, that is. I was ecstatic the day I traded that car in on a 4x4 pickup truck!