Pull the valve cover back off and place it on a flat surface. An oak desk would be a good place. Check for any warpage. If you can slide a match book cover under it at any point then it is warped too much and will leak. Have a machine shop mill it down for you if this is the case.
Otherwise look for a crack in the cover. You can use a bottle of food coloring to do this. Mix a gallon of water with one bottle of food coloring. Immerse the cover and let it sit for a few minuets. Rinse it just enough to get the majority of the food coloring off and look for any lines. They will be cracks. Replace the cover if this is the case.
Please post any other concerns here.