Not all caps have the return valve, so when you look for a new one check that it's got a brass "cap" in the middle. The spring is pretty light, so you should be able to get a fingernail under the edge and pull it up. Some look like they are right, but the brass is a big rivet to hold the bottom gasket.
BTW, I answered this question about 6 months ago and I never, ever throw away a graphic.
