A good pen flush can be made by mixing ammonia and water. 10% ammonia should do. Also, if the piston doesn't want to move, don't force it. It's pretty easy to break something with just a little too much force. If it needs a new cork or seal, you'll need to send it off to a restorer. These old things are a bear to take apart without ruining anything - and they're worth enough money that you want to do it right.
If it's a newer model, not celluloid, you could send it and $80 to MB and they'll put your nib into a new pen and you'll have basically a brand new pen. If it's really old, celluloid, then you don't want to do that.
Got any pics to share of it?