Actually have the old gasket here in front of me, it's coated with "crystalized" coolant and aluminum corrosion. The rubber has that "perished" look, perhaps due to heat and age (15 years worth) but otherwise intact. It's possible it lost some of it's sealing quality and allowed coolant to flow around it. There's always been some of slight coolant loss in the past, tightening clips and hoses helped, but I never managed to stop it completely, this is probably the reason why..
