Anyone had experience fixing fluid leak from brake booster box? Big / small job?

 

When you say "box" what are you referring to? If you mean the booster (the round cylindrical part up against the firewall), it should not have fluid in it. If it does, it is probably brake fluid and the diaphragm in there is toast. You would need a new booster.

If you mean the brake master cylinder/reservoir (the rectangular part where the hard lines connect to, and the fluid is added), it really depends on where it's leaking.