I agree w/Mongo - Replace the ENTIRE Refill Valve. (This is the ENTIRE 'stack', 'ball', and 'cap' ...)
You see, when it comes to plumbing, with DIY plumbing, old parts don't work too well w/new parts. Do be careful when you begin 'detaching' the Fill Line (that is hooked up to the shut-off that comes out the wall) in that you don't bend, strip, or break anything. If it's on too tight and you're not comfortable removing it from the Refill Valve, get some help. Sometimes, you need a monkey wrench as well as an adjustible one.
Read the instructions that come with the part for adjustment info and you're on your way! (I start the adjustment by 'matching' the old one w/the new one, side by side.) If it makes matter, stay away from the Refill Valve that has an Anti-Siphon Kit. Personally, I they don't work very well for me when it comes to installation although I DO like their 'purpose' in life ...)