When the vehicle is turned off, the brakes can't be pressed, and when it accelerates at a constant speed, it is accompanied by air leakage. It feels dangerous. The 4S shop said it was normal.

First of all, this is a fault, not normal as 4S said. When you said you couldn't step on it, you should not step on it now. The sound of air leakage also shows the root of the problem, that is, there is something wrong with your braking force, because the braking force is realized by using the vacuum behind the intake manifold. When cruising at constant speed, the power is reduced because the vacuum behind the intake manifold is reduced because the throttle is turned on. So let 4S help you check the position from the intake manifold to the brake vacuum booster pump to see if there is any air leakage. In addition, if it is as I said, there should be other fault manifestations-it is not easy to start when starting, and the speed is too high when idling.