Divided parts

[Panteck]

I'm writing a piece that has divided parts. My problem is that Lilypond is 
putting brackets around every line, not just the lines where the divisi 
occurs. I though that the SystemStartBracket.collapse-height property would 
take care of this for me automatically, but even when I try overriding the 
value, I still get the brackets. Does anyone have any suggestions on how I 
can get rid of the brackets on the single lines, but keep the brackets on 
the divisi line? I'm using Lilypond 2.7.10 on Windows native.

Thanks!

--Steven

partI = \new Voice {
   \clef bass
   a,4 b, c d e f g2 \break
   g4 ^\markup { "divis." } f e d c b, a,2 \break
   a,4 ^\markup { "tutti" } b, c d e f g2
}

partII = \new Voice {
   \clef bass
   R1*2/4*4
   a,4 b, c d e f g2
   R1*2/4*4
}

\score {
   \new StaffGroup {
       <<
           \partI
           \partII
       >>
   }

   \layout {
       indent = 0\cm
       \context {
           \RemoveEmptyStaffContext
           \override RemoveEmptyVerticalGroup #'remove-first = ##t
       }
   }
}