A trailer with no spring parking brakes is parked using only service brake air. Why is this unsafe even if the brakes initially hold?

1. Leaks can bleed off the applied pressure, allowing a rollaway unless wheels are blocked
2. The trailer will build unlimited pressure by itself
3. The tractor protection valve will refill it forever after uncoupling
4. The low-air warning on the tractor guarantees the trailer will stay parked