The r528 CAP is the "seal" in the korky/hunter 528.
Original poster: Terry recommends replacing that R528 cap if you have that Korky valve. You can be confident doing that.
If you were paying $100+ for a plumber to come out, then, hell-yeah, might as well spend the extra $10 for the whole valve to be replaced, although you wouldn't pay Home Depot parts prices to a plumber, who will have to mark that part up to account for the cost of inventory.
But as a DIY project, go ahead and replace that cap first and see if it doesn't solve your problem.