Both front tires lock up ? Tho normally master cylinders dont cause locking like this unless the plunger inside is sticking or air in the lines. If it was sticking then the brake peddle would have a lot of play in it when the brakes lock up. Poss that your getting sort of a vapor lock on the brakes. Does it do it under light braking or just harder braking ? Take a few mins and a quart of brake fluid and flush the whole brake system. There is a bleeder on the bottom of the master as well.
