First of all, I suggest you check whether the piston of the brake cylinder is stuck due to dirt. It is also possible that the pipe that fixes the brake cylinder is stuck, causing the brake cylinder to fail to return. The pipe is in the part where the brake cylinder and the suspension are fixed, outside the fixing screws of the brake cylinder and the suspension, that is, in the holes of the two fixing bolts of the brake cylinder. You can remove the brake cylinder, remove the tube, sand it with fine sandpaper, apply butter, and finally put the cylinder back into the vehicle.