impotent "space-before.*" in attribute set component.title.properties

[Erik Leunissen <e.leunissen@hccnet.nl>]

I'm customizing the appearance of chapter titles using the attribute set 
below in my customization layer.

No matter to what extreme values I set the "space-before.*" attributes, 
they don't make any difference at all in the fo output (PDF). This as 
opposed to the "space-after.*" attributes, to which the output is 
sensitive as expected.

I'm clueless about what's going wrong: my logic, xsl or fop processing, ...

Any help in directing me where to look for something inappropriate is 
much appreciated.

Thanks in advance,

Erik Leunissen.
--

Processing system
-----------------
docbook stylesheets version 1.78
FOP Version 1.1

Customization
-------------
<xsl:attribute-set name="component.title.properties">
  <xsl:attribute name="font-size">
    <xsl:value-of select="$body.font.master * 4.0"></xsl:value-of>
    <xsl:text>pt</xsl:text>
  </xsl:attribute>

  <!-- The following lines don't have any effect at all -->
  <xsl:attribute name="space-before.minimum">12.0em</xsl:attribute>
  <xsl:attribute name="space-before.optimum">13.0em</xsl:attribute>
  <xsl:attribute name="space-before.maximum">14.0em</xsl:attribute>

  <!-- These do affect the output -->
  <xsl:attribute name="space-after.minimum">2.5em</xsl:attribute>
  <xsl:attribute name="space-after.optimum">3.0em</xsl:attribute>
  <xsl:attribute name="space-after.maximum">3.5em</xsl:attribute>
</xsl:attribute-set>

-- end of message --