The Rancilio Silvia is a robust machine if you look after it. It's easy to fix if anything does go wrong and parts are pretty easy to come by.
One thing that does happen over time is that the Steam/Water valve can become "lumpy" or "sticky" in operation and not smooth like it should be.
To fix this is quite simple. Do a normal de-scale operation (use the instructions based on the descaler you're using). Whilst the tank contains descaler, run some descaling fluid into the tap (open the tap as wide as possible). If you can try and leave the descaler in the tap (maybe turn the machine off after it's up to temp) so the descaler can sit for a bit with the tap open.
What this does is let the inside of the tap get descaled. I've found that on several occasions this has returned the tap back to operating as smoothly as it did when it was new.