I had occasion to step hard on the brakes (linked together) of my year-old 1726E. When I released them, they stayed on. The pedals did not come up and the brakes were in effect. It was not that the parking brake lever was in the wrong position, holding them down. I repeatedly stepped hard on the pedals, and finally the they released. But this could be bad in some situations, and I don't want it to get worse. Any idea what might have happened to cause this?