Have you tried a stopleak fluid? Mine had a slow leak for several months, 'slow' as in just enough to keep the drop arm looking wet without actually ever dripping on the floor or needing a top up.
If it was up to me, I'd have left it like that but an imminent MOT forced my hand. So one bottle of Wynn's, a few sprays of degreaser and a jet-wash later and she was leak-free enough for an MOT pass.