I wish i had a good answer as i have the same issue. I called McCulloch/Husqvarna and they sent me a slew of parts to change including the tanks (waiting on one that;s on backorder) hoses, tank selector valve thing, o-rings and a few other parts.
I have yet to replace the parts as i just received them. If it's less than a year old i suggest calling them at 1.800.521.8559. They only asked me for the date that i purchased it and the serial number, so you may be able to get parts for free, if out of warranty.
The link below is for the parts breakdown.
http://ipl.ordertree.com/ipl/604/FHH18A.pdf
I will add another post once i tackle the replacement parts project.
Good luck!
Rob, June 2009