You need to buy the cap and o-ring replacement set. They are only a few bucks a piece at the parts store or you can order them on e-bay through Mr. injector, I got mine in 2 days for them. Kit comes with cap, o-rings and new filters, for like $16.
I have seen the kit that Liquid is talking about, but I figure if you have them out why not replace the caps too. The caps will be a bit of a pain to remove sometimes, but if you soak the new ones in hot hot water, they will be easy to put on