From new member Diane.
The following was from a private message. I haven't checked in much lately so this post is about three weeks after it was sent. I went to the link and indeed the X-rings are there. I may have to try them. Thank you to Diane for the helpful info!
<I was trying to add to your comment about the ballcock with leather washers on a 1928 Case toilet but somehow I can't get permission to do so.. I recently had occasion to repair a similar toilet from 1941, but the ballcock looked the same. One of the leather washers on the inlet plunger was gone; the second didn't hold back water at all. I couldn't get a leather washer and didn't have the tools to cut new ones from scratch. I tried an O-ring, but it caused the plunger to bind.
The magic that worked perfectly was something called an X-ring or a Quad ring. They're basically like an O-ring with an x shaped profile. They don't twist and each has two thin sealing surfaces. I bought it from McMaster Carr in a 1/8 inch width, 3/4 inch OD and 1/2 inch ID. It didn't leak or bind. Unfortunately you have to buy a bunch of them, but it's worth it. McMaster-Carr
Please post this to help others who want to save a toilet that probably matches other porcelain pieces.>