The lever travel had diminished recently, but still ok, so I removed the master cylinder to investigate. The master cylinder looked good, and I ensured the holes were clear with brake cleaner and a burst of compressed air.
![Image](https://shop.maxbmw.com/fiche/Diagrams/B0003664.png?v=09082020)
It looks like my 13mm MC is missing a couple of bits, specifically the seal and the washer at the circlip end. The two seals are on the internal cylinder shaft. It's been like that for years.
With the master cylinder in hand, it looks like the circlip goes under the washer. However the fiche shows the circlip outside the washer. I'll take it as one of those anomalies we see occasionally with the fiche.
![Image](https://photos.smugmug.com/Brembo-F08-rebuild/BMW-brakes/i-PKjMJnX/0/CCf9G8WMhdPP2cBNZ3Br7jKvGGp95WSJgt6jb3WXP/M/front%20mc.-M.jpg)
To me, it looks like the circlip goes under the washer. The washer looks too big to fit into the cylinder (edit) and the one I found doesn't fit in..
Mine looked like this when it was removed. There is a circlip under there. Just no outer seal or washer. It cleaned up just fine.
![Image](https://photos.smugmug.com/Brembo-F08-rebuild/BMW-brakes/i-bqHLwPt/0/CscvmFs5hxd5kR5k48cVQWmKt3MrTcgRBzLmPJC8M/X2/2024051300393941-844819465248271361-I-X2.jpg)
So, pending further thoughts, I've reassembled the lot as it was. Adding the seal and washer won't take long when...
addition: I've found a washer amongst my parts stash (brake section) and will look for a seal that fits. I don't think I'm going to be too fussy on that, as long as it's snug enough and wide enough.
I'm still unsure why the travel has diminished recently. I don't think this explains it. I'm going to look at the ss hoses that are on the bike. Maybe the lining has deteriorated? The right side flushes ok from the top, but isn't accepting a retrograde flush.