\score {
  \new StaffGroup <<
    % Must be lower than the actual number of staff lines
    \override StaffGroup.SystemStartBracket.collapse-height = 4
    \override Score.SystemStartBar.collapse-height = 4
    \new Staff {
      c'1
    }
  >>
}
\score {
  \new PianoStaff <<
    \override PianoStaff.SystemStartBrace.collapse-height = 4
    \override Score.SystemStartBar.collapse-height = 4
    \new Staff {
      c'1
    }
  >>
}

\paper { tagline = ##f }