When you see leakage during brewing it’s time to replace your E61 group gasket.

A silicone gasket is the best choice as silicone is soft, meaning you get a better seal with less pressure. Also, silicone is durable, and likely to outlast 10x standard rubber gaskets.

These are 8.5mm gaskets. They fit all machines with 58mm E61 groups.

When you change your group gasket I recommend changing your group shower screen as well - upgrade to an IMS Precision Shower Screen