I had a brake fluid leak on the '94 525i that I sold to my mother. After careful checking, it was coming from these same two rubber plugs. The old ones were hardened and deteriorating. Whenever I wiped them, I wiped off brake fluid and black rubber. I put new ones in and am anxious to see if that does indeed stop the leak. It sure wasn't leaking anywhere else!
Hope this helps,
vince
vince