Hi Abalieno,
you probably need to tweak your input formats. Go to administer -> input formats, and either turn off line break converter for the format you use, or create a new format with line break converter turned off.
Okay, but it's still a broken feature. A <pre> block should ALWAYS bypass the line breaks. In every condition since it's exactly how the html tag is defined. I can disable the line breaks for the WHOLE node but I'd like Drupal to keep doing that and bypass the line breaks only when dealing with the <pre> tag.
This is supposed the default behaviour and should be considered as a bug.
-HRose / Abalieno