If your espresso machine doesn't have a 3-way valve then it has a group valve.
Espresso machines will often leak when you steam or pull hot water from the steam wand. What you're seeing is normal for any machine without a 3-way valve. In it's place such machines use a group valve; a one-way valve that permits hot water to flow to the grouphead when a shot is pulled, but keeps coffee & grounds from being flushed back into the boiler before & after a shot is pulled.
The group valve is rated at ~1.4 bar. The OEM steam thermostats are set at ~300°F, a temp that corresponds to ~4 bar. As you can see, the pressure generated when steaming will force hot water & steam back through the group valve & grouphead.
Here's a link that explains it in detail.
I get around this situation by using a blank back flushing filter basket when I steam.
