First, look and see if interior of wheel cylinder is pitted. If it is, probably time for a new one. Hone using brake fluid as the lubricant and you might be able to save it. Check the plunger also as they deteriorate and become pitted also. Buy a wheel cylinder repair kit. Here's a start but, lots of places sell the parts.
https://www.ebay.com/b/Brake-Wheel-C.../bn_7063924290