In a vehicle or trailer that is equipped with air brakes, the service brakes work by air pressure. The problem with using service brakes as parking brakes is that over time, the air pressure may bleed down and may cause the brakes to release. Most trailers equipped with air brakes also have spring brakes, which should be used as parking brakes. If your trailer doesn’t have spring brakes, apply the trailer service brakes and block (chock) the wheels in case the trailer service brakes release.