having header bottom line with header.rule

Hi all !

I'm new in docbook and try to customize my pdf output, generated with
fop.

I succeeded in customizing some things for footers and headers like
content depending on type of pages (titlepage, book page, set page,
etc.) or layout with footer.table.Now I have problems with the header :
when I set header.rule to 1, nothing happens, I can't see the bottom
line.

I have set this at the bottom of my stylesheet, with others parameters :

  <!-- activating headers & footers rules -->

  <xsl:param name="header.rule" select="1"></xsl:param>

  <xsl:param name="footer.rule" select="1"></xsl:param>

It works well
with footer.rule : line appears and disappears when I set 1 or 0, I
also can change it by changing border width in foot.sep.rule.

But with header, I can't manage to do anything : header.rule has no
effect, nor head.sep.rule which I copied from nwalsh/pagesetup.xsl to
my custom fo.xsl. I've also tried to remove the test <xsl:if
test="$header.rule != 0"> to force the line on headers, but still no
change on headers...

I haven't touched the header.table and head.sep.rule but even when I
copy them in my file, nothing change...

Has anyone an idea of what I am doing wrong ? (or what I am not
doing...)

thank you very much in advance.

Marie.