In the first instance of this problem, my brake pedal was stuck to the floor. By pumping it a few times, it gradually released and rose to the proper level. Brakes then seemed to work fine for a couple of weeks, until I reversed out of a parking space, but could not brake the car when I shifted into drive. I pushed the brake pedal to the floor, but could not stop until I ran over two large boulders that, thankfully, stopped the car short of a big tree.
My mechanic doesn’t know exactly what went wrong, but is replacing the brake cylinder. I fervently hope that fixes the problem! What could it be?