My Tefal Pro express 2929 blows steam from base station?

I have been asked to have a look at the above iron for an elederly friend. When I top it up and switch on it blows steam/water from the pressure release valve in the base plate and nothing from the iron. I dont want to tell them to buy a replacement if it can be fixed. I am an electrician so no problem stripping it down etc
Any ideas welcome.