How do I fix my polti vaporetto 1500 that constantly outputs steam?

Hi
I have a polti vaopretto 1500 that whenever is switched on and is producing steam constantly outputs steam from the gun if the trigger is pressed or not.

I have tried a different gun with no difference.

It only seems to happen once the trigger is pressed in for the first time at use.

It also occurs if you disconnect the gun, steam comes out of the plug for the gun. The steam stops immediately if you switch the cleaner off at the wall

Thanks for any advice
Jon