Staff.voltaSpannerDuration

[Werner]

Hello,

I wrote a staffgroup with two voices in two staffs and harmonys (chordmode).
There is a repeat with alternatives over 8 bars. Want to shorten the volta 
bracket.

I tried without succes. (No warning, no problem while lilypond is formatting,
but it always is 8 bars long.)

I put the \set Staff.voltaSpannerDuration = #(ly:make-moment 4 4) command once
inside every voice.

Any hints?

Thanks

Werner