1290 SD GT master cylinder failure

I've got an odd one here. I parked my 2016 SD GT in late November 2019 and had to back it out of the garage in early January. I noticed the front brake lever was really soft. Then in late February, I had to move it again and the front lever when straight to the handlebars. This front master cylinder was subject to the 2018 recall and I had it done at the time.


When I parked the bike, everything was fine. There is no leaking fluid visible anywhere in the front brake system. Worried that the recall repair had failed somehow, I purchased a RCS19 master cylinder and intended to swap it out. However, it dawned on me to try an bleed the master cylinder before tackling that project. Sure enough, there was a lot of air in the front MC. After maybe eight cycles of squeeze, open, close and release, the front brakes are now back where they were when I parked it.

I am stumped as to how air got into the front MC while it was just sitting there doing nothing and that there are no leaks. Any words of wisdom out there? Should I return the RCS19, or install it anyway? Part of me is pretty nervous that the front MC can crap out like that, especially when I do a few track days (for fun) each year. Thanks in advance.

Although I haven't had this happen on my 1290 SDR (perhaps because it never sits for more than a day or two), I have had it happen many times on my 350 EXC-F. Brakes would be perfect when the bike was parked, come back to it a few weeks later and the lever comes back to the bars. No leaks. I found that I could get some brake by pumping the lever several times rapidly but, after not moving the lever for 10 seconds or more, there was no brake again. I found that if I pumped it enough to get some pressure and then tied the lever pulled in and let it sit overnight, then the brakes would be fine again, although I might have to repeat the process once or twice to get it perfect. I got in the habit of tying the lever back whenever I was going to let it sit for more than day or two and haven't had the problem since. In fact, after doing the the tie back thing for a year, I got lazy and stopped doing it and the brakes have stayed good even after extended sitting since. So, I don't really know what's going but I wouldn't worry about brake failure once they start working again.

Try bleeding at the master cylinder fitting, sometimes you can just put some towels on it , hold the lever in and open the bleeder nipple quickly. This will burp the system, just did this myself.

Mojodog and redtale2000, thanks for your replies.

So after I bled the front MC, I took it out for a gentle shakedown. The front brakes were fine, but then I noticed the rear brake was soft. Given that those are separate circuits, I'm starting to wonder if air is entering the system via the ABS pump/modulator. What else could affect both circuits with the same issue? Or should it be my expectation to bleed both MC's periodically? If so, it would be the first motorcycle I've ever owned that required regular bleeding of the brake system.

Thanks!
Jon

yes, i do full bleeds once a year and "burps" in between.

Well, I made an executive decision over the weekend. The front brake lever started to get just a little soft again, so I went ahead and installed the 19RCS MC. Given that the old MC was Brembo, the bolts lined up perfectly for my right mirror mount and the OEM brake switch slotted in perfectly. The only tricky part was finding a new way to mount the reservoir because there was nothing wrong with the old one. So I bought a generic universal 2-part mirror mount which came threaded at 10mm. Lucky for me, the rubber-clad hole in the mount for the OEM reservoir is also 10mm. Serendipity...

While I had the Capri pneumatic bleeder out, I flushed the rear brake fluid, which improved its pressure a bit. The new front MC is much firmer and is now ready for a shake down.