Variations in cooking times with pinto and black beans are usually attributed to age of the beans, elevation and even how hard the water used for cooking is in your area. Adjust cooking times as needed for the beans being used.
Be sure to rinse the pinto beans and pick out any small stones. (In all my years of picking over beans I've never actually seen a stone, but it can happen so better safe than having to run to the emergency dentist during the game.)
Do not presoak the beans.