This is a notorious problem with the Hoover V2 SteamVac's. In fact the tank design by Hoover on your SteamVac was so bad, that Hoover redesigned this tank almost immediately due to the numerous problems customers were having.
For your particular problem, you really have two options. The first option is putting a bandaid over the problem, but will still fix it. We sell the entire gasket assembly on the bottom of the detergent tank as a replacement part. Unfortunately it's sold together with the seal, spring, and valve together as an assembly;
You also may wish to fix the problem completely by replacing the entire clean water solution tank. Hoover has redesigned this tank, and the new one I don't see near the amount of problems that I do on the older style tanks. The plastic is much more sturdy, and less prone to having the gasket and valve issues that the old tanks do. It's important to point out that the new tank does not come with the handle. You'll need to remove the handle from your old tank if you buy a new tank assembly complete. Rest assured that if you do purchase any part from GoVacuum, you'll have 60 days to return it for a full refund, just in case the repair doesn't work. Not only do we not deduct restocking fees or shipping charges, but we also include a return label with every order that you may use to return any item to us, and we'll even pay for your return shipping!
Here's the Hoover SteamVac V2 Clean Water Solution Tank, Part 12002549
If you'd like us to do a repair photograph series on this repair of the valve and gasket assembly, please just request a repair clarification here on Fixya, and we'll be happy to do so. If you found this helpful, please be sure to rate the review as helpful by clicking on the 4 Thumbs Up Icons in the top right hand corner of the repair.
Thank you so much,