Ive seen this a few times over the years, usually as I recall it is one wheel or the other but not both. In my case to both wheels stay locked up (brakes on) even after bleeding off all the air pressure and with the brake valve in the off position. after 30 seconds or so with all pressure bled off the air tank the brakes finally released. I pumped it back up and they were again locked up without even applying any brake valve (UP 6). I once again let all the pressure out of the tank and after thirty seconds and a few applications of the brake valve on and off they released ? does anybody have a good idea whats happening and how to prevent it ? This all happened after the first flight this morning where they worked completely normally and they worked propery three flights last weekend.