Brake Hardware kits is all the springs for both sides, costs ~$11
Self-Adjuster Kit is all the star wheel, cable and lever for ONE SIDE Only ~$13
Wheel Cylinder Rebuild Kit is ~$4
My rear wheel cylinder was leaking, but all the seals looked fine. I suspect the cylinder got coated in gunk, because it took forever to hone away all the gunk on the cylinder, but it was the gunk that caused the seals to leak.
I flush out all the old brake fluid and replace with new by bleeding the brakes once a year. I've never had a leaking wheel cylinder in vehicles I've owned for a dozen years. Leave the brake fluid in there for 10 years and even if the degraded performance isn't enough to hurt you, the dirty fluid will gunk up the system and you'll get leaks in rear wheel cylinders or sticking calipers up front.
Its only old vehicles I've bought or help friends and families with, that have leaking wheel cylinders or sticking calipers. They never change the brake fluid.
Changing brake fluid once a year, or even every couple of years, will do wonders to increase the life and reliability of your brakes.