When I had a Classic it happened all the time. I had my machine for 4 years. It was never really a drama for me. I just gave it a really good purge before I steamed. Not sure if you can replace the seal in the steam valve as I never pulled it apart; although I wouldnt be surprised if you could.
The other thing is that you actually make matters worse by overtightening to compensate for the leak...eventually the sealing surface suffers more damage and the problem escalates (same thing happens with your domestic tap washers..)