Is moving the bike essential to the problem manifesting?
If you pump up the lever and then walk away for 10mins with the bike stationary, does the pressure still go?
If so and you can't see any leaks then I would suspect master cylinder seal, it may have taken a huge pressure hit if the bike went down on the front brake lever. You could leave the reservoir open, keep repeating your pump up and pressure drop and see if you lose fluid, if not then master cylinder.