Can you be a bit more specific? You say the dump valve does not close completely? Does that mean the actual valve is not seating correctly, does it mean the handle is bent and won't slide any more? Does it mean you are closing it as far as you can and it's still running on you?
If closing completely, you might try getting a round shaped brush, see if you can reach the actual valve and give it a good scrubbing. You've got nothing to loose since you are considering replacing it anyway? If it's closing all the way, it's possible it just has crud on the seals. And then again, it's possible the seals could just be worn out too. But a good scrub brush on the end of a stick, swirled around the seal won't hurt anything in your situation. Try it first. See what happens.