Had small leaks in both tanks. Did a fuel flow test. Started with 10 gallons per tank. Found that in a level attitude, both tanks had almost 5 gallons of unusable fuel. This combined with the lack of fuel drains in either tank, it was time to improve the system. A previous BD-4 I owned had wing tank sump similar to the ones used on the new BD-4 C. These work amazing, less than one gallon unusable per tank. Little risk of unporting a line, a low point for Watertown collect and a drain to clear water and sediment. Top wing skins opened up, tanks were in excellent condition. Blocked off the rear pick up in both tanks. Then added a tube to the Center pickups, running two thirds of the way down into the underwing sumps. Screens were added to the pickup tubes. Made a mold for the underwing sumps. Two sumps we made from the mold using West Epoxy System. Sumps were riveted and proseal Ed into the bottom of each tank. Then the tops of the tanks were repaired. Came out great. The unusable fuel in a nose up attitude is about half gallon in each tank.