The bigger question is why it's blowing. I've had my car to and a tick over 100*C on the gauge without popping the cooling system. Maybe your pressure cap isn't releasing system pressure above 1.4 bar as it should. BTW, that reservoir isn't an overflow tank since it's part of the pressurized system. There is an overflow tank in the right fender behind the wheel well liner.

Sixto
87 300D^2