Soak chestnuts overnight in water. Wash brown rice and sweet rice at least 3 times or until water is clear. Place rices in pressure cooker. Drain chestnuts, saving the soaking water, and add to rice. Measure 3 cups of water including the soaking water from the chestnuts and add to rice. Add salt, fasten cover, and bring up to pressure. Place heat diffuser under pressure cooker, lower flame, and cook for 50 minutes. Remove pressure cooker from stove and let pressure come down.