Grease or oil on the rotor or pads, even in small amounts, will cause the brakes to 'grab' which may feel like a lock up situation at low speeds. Make sure to clean them thoroughly with CRC Brakleen after handling. It's counter intuitive that oils make brakes grab quicker, but it's true. I wouldn't completely discount the other possibility of the rubber line either, old lines can be fragile, and just moving the caliper out of its original position could cause the inner lining to separate. Rubber lines are cheap and easy to replace, you're in the rust belt so a little extra care might be needed with the fittings. I always buy a new banjo bolt just in case, and use a line wrench on the hardline fitting after soaking with PB Blaster several times.