By "dump valve" I assume you're talking about the blow off valve... in which case if you take it out of the car you'll see there are 3 small phillips-head bolts on the bottom - undo them and the lower part will come off. In there you'll see a hole about 2mm diameter.
Find whatever you can to plug it, and make it
very snug. People frequently use a screw and add epoxy to be safe. It's very easy to do
