Does it always drop to zero or does that only happen the first time you open the steam wand after turning on? Once its dropped to zero after opening the steam arm, when it comes back up to pressure does it operate normally? If so then your anti-vac valve is either stuck or non existent.
Are you ok with opening the case and looking inside? Have a look at the valve when it's cold and press it down. If when you press it it falls then its getting stuck and not releasing false pressure in the boiler on start up. This can happen when it gets old or after a descale as the rubber seal gets old and sticky.
If it happens all the time - not just after start up - then I dunno!!!