FYI for our fellow Westy seat owners. I followed the excellent picture in this post:
https://sprinter-source.com/forum/showpost.php?p=261616&postcount=5
In the case of the Westy, when I pulled the bulb out, I was able to find three pieces: the two black round plastic bits and the little round white rubber flat piece that gets squeezed between them. I didn't find a long, square white piece as pictured. What I did was assemble the three pieces in the order shown, with respect to the correct orientation facing the rubber bulb. I put these pieces in the hard tube protrusion that remains in the Westy when you pull off the bulb. By placing the pieces in this hard tube in the correct order, I was able to then slip the bulb back onto the hard black tube. Now it holds air.
This sounds worse than it is, and requires no disassembly of the seat other than carefully pulling the bulb off. It takes about 5 minutes. For some reason it overwhelmed me and I delayed having adjustable lumbar support for years!
Also an interesting note: our passenger seats in the Westy do not have the air-lumbar support feature, as regular Sprinters appear to. At least that is true for mine (#133). Not that it is so necessary as I find the seats to be generally quite supportive.