The previous paint used on the cylinder had been a type of red oxide but it is not specified to be able to stand up to the high temperatures encountered when pressurised steam is involved. I thus did some investigation and found a coating made by T&R Williamson Ltd who for many, many years have specialised in supporting the railway and other industries. They come well recommended and were very helpful on the phone.
|New finish with the piston retaining ring in place|
|Side view with condensate drain valve disassembled|
|Valve ball and housing...|
|...with a view into the valve chamber|
|Screw-in plug with hole through it|
When steam or air is allowed into the cylinder under pressure, the ball is blown on to the tapered hole in the plug and prevents the steam or air from escaping. This maximises the force on the cylinder's piston and the loco's brake. (Also on YouTube).
When the pressure is released again the ball drops back into the chamber to open the hole again and allow condensate to drain. (Also on YouTube).
So that's how it works, I hear you say!
Post script 22nd June 2012: I tried a 0.75" ball instead of the 0.625" ball used in the videos. 0.75" is too large such that it blocks the outlet when the screw-in plug is tightened. I don't have a smaller ball to try so 0.625" it is. In this case, clearly, size matters!