Brian ... we have the same problem with a Hitatchi top loader,
I've pulled the top off and there is a very heavy layer of black gunk caked on the inside of the outer bowl. There is a gradual build-up down from the top till it is about 8mm thick in th middle then it thins out again, scraping is the only fix!! The machine is about 6yrs old and we have been told fabric softener is the cause. Before this we tried epsom salts with no luck.I too thought this was a leaking seal till I pulled the top off!!
Graham
January 2005