I have a Hoover steam vac that did the same thing. A small plastic valve (called a "quick disconnect assembly" on my parts list) that costs about $3-$5 that gets activates when the upholstery/stair cleaner attachment is inserted is leaking. It is supposed to be activated with the suction when the attachement is inserted and closed when no attachement is inserted. If the valve has gone bad, it will leak as soon as the water tank is put on. At least that is what mine was doing. I have replaced the valve twice in about 10 years. Hope this helps.