If you have a firm lever(ruling out air in the lines) it could be that your pads are glazed. This has happened to me before. You can take some sand paper on a flat hard block and sand the shiny glaze off them. Make sure you thoroughly clean them with brakleen first, and after. I believe I...
First off, I don't know anything about doing an inverted fork conversion besides the fact that it looks cool. I imagine you could measure rake, trail, etc and if you keep it similar to the stock values it should handle similarly stable(with improved damping due to the modern forks). If it's not...