I had a Cadillac come in on a vacation trip. The car stopped coming off of the freeway but the brakes wouldn't release. It turned out that the customer had taken the car in to be serviced before the trip and the person who changed the fluids and top them up put power steering fluid into the master cylinder with seal swell and it damaged the entire brake system. We had to replace the master cylinder, propositioning valve, calipers, wheel cylinders and brake lines. Everything with rubber in it. That could be a possibility. Drain the fluid and make sure it's not contaminated. Also check the brake lines. Brake lines can collapse on the inside and will let pressure through but won't let the small amount of pressure to return. It could also be a problem with the abs system applying the brakes.