DEFLATE after Substitute on Apache Proxy Server

Question

I've got a problem in configuring Proxy on Apache. I want to:

1. get a content from a website (Deflate or not) ;
2. apply a filter on it to substitute some text on the body of the page ;
3. resend the page to the client after apply the Deflate filter to the altered page.

My configuration is like this:

ExtFilterDefine FAnalyse mode=output intype=text/html cmd="path_to_my_script"

<VirtualHost *:80>
    ServerName site1.com
    <Location />
        SetOutputFilter INFLATE;FAnalyse;DEFLATE
        ProxyPass http://site2.com/
        Header append Vary User-Agent env=!dont-vary
    </Location>
</VirtualHost>

It works fine without the DEFLATE filter but the content is not compressed.
With the DEFLATE filter, the client seems to get the compressed content but with bad headers because displays a page like this :

‹í39 Èæ*ô2ÃA(„ºÒªíïÄÃ5¹Q°sç¿^Þ^N.ç7Æðj#?
æÈ>ñ<ÙæýåÕßÑÍù{°D?]ßÎ~ ty&,锓qÏ:<¬Ð™àYR
lŒŒÓ@À˾íQÿqâõ¬ |ë6%^ç($aÅ ‚˜¸¬„ŠGàûqEyð

My script is not the problem because I substitute it by a Substitute Filter from Apache and the problem is the same.

I tried this directive :

Header append Vary User-Agent env=!dont-vary

without success.