The plastic becomes brittle / weak with age and the plastic fails, the theory is high pH anti freeze can accelerate this problem. I know the Mercedes anti freeze is buffered for low pH. (they also use plastic tanks). You can get low pH. anti freeze at your parts store, it is usually orange or yellow colored as opposed to the high pH. green (cheap) anti freeze. Irregardless once the damage is done replacing the part is the only long term solution.

Tim