After going through 3 sets of replacement plunger corks, none of which were leakproof, and almost impossible to slide as well, I broke down and bought a set of beautifully machined-from-brass o-ring plungers from Baxter Cycle in the US http://www.baxtercycle.com
Each unit is $20 US and comes with some sort of o-ring lubricant for inserting into your petcock. The first thing I learned is that the plungers/o-rings were too loose to properly fit into the cheap Chinese knock-off petcocks I had purchased a while back. Fortunately, though, I still had my original Ewarts petcock saved in the parts bin. The plungers fit perfectly, require almost no effort to open or close, yet neither has leaked a drop so far.
My only complaint is that they didn't provide a set of replacement o-rings, just in case, as I had already found that size is very difficult to come by with my local bearing/seal house.
If I were ordering again, I'd probably ask for a spare o-ring with the deal. Otherwise, I highly recommend for what looks like a long-term solution for a perennial problem.
Here's the direct link to the product: https://www.baxtercycle.com/product/push_pull_petcock_plunger_insert/