How exactly do you use RPN in prv?

I'm guessing adding expression can be done by adding RPN, but every time I try to use it it messes up the volume of the whole track.

Even when I haven't done any work each note already seems to have RPN value relative to its velocity. hm..
Anybody can share tips for using RPN effectively? I'd really appreciate it.
 
I'm trying to add expression to my violin and cello in an orchestra piece.