I realize there have been a couple posts on this topic, but I thought this new info warranted a new post. After experiencing this issue "spongy rear brake" on my 2018 R1200GSW I mentioned it to my nearby dealer in Denver and to my surprise they said they would replace the rear master cylinder under warranty just based on my description. When trying to apply the rear brake the pedal would travel more than 2" before any braking would occur and the braking power was very weak. Front brake was not affected. Once they replaced the part and bled brakes I was astonished how well they now worked. The mechanic said BMW had done some redesign on the master cylinder to remedy the air leak issue. I would encourage anyone with this issue to contact their dealer. I'm a little surprised this has not been a recall item as brakes are a pretty important safety item. This may apply to other year WC GS's.