Most 400/4 owners that I know have had some bother freeing up seized bleed nipples. The reason for this is the old dissimilar metals/salt/electrolysis that we all know about by now.
My hot tip is to use PTFE tape that plumbers use. This not only prevents the bleed nipples seizing up but it makes bleeding the brakes easier and less messy because brake fluid can't sneak past the threads.
