Why mess with the valves? They are pretty expensive and unless you have your yak severely over-loaded you won't need them (you'll have more serious problems with that). Have you read the previous replies? All the experience on this board and they are telling you plugs are not needed . The only scupper plugs I keep installed are the two under my seat. As a result I rarely get wet at all, a wave comes over and drains immediately thru the scuppers (as they are designed to work).I'm going to look for the scupper valves. I guess they can regulate water level in the yak.
Wow, excellent suggestion and details on how to make!..............MUCH cheaper option for scupper plugs.. Go to Academy and buy a pack of yellow foam practice golf balls. Drill a 1/8 hole straight through. Take a 9-10 inch piece of paracord, double over and tie overhand knot at two tag ends. Push loop end through golf ball and tie another overhand know right next to ball. Make knot as tight to ball as you can to create some complession on ball. it makes them watertight. You can make 6 scupper plugs for about $5. Cheap, bright yellow and they float!