|1. A reverse pressure greater than that in the intended normal direction and/or pressure of flow or thrust
2. An escape of liquid by virtue of the physics of a simple siphon.
3. Pressure in plumbing pipes that is less than atmospheric pressure.
4. The flowing back of used, contaminated or polluted water from a plumbing fixture or vessel or other source into a negative pressure in such a pipe.
5. A pressure in the supplied system which, for some cause, becomes greater than the supply pressure (pressure above atmospheric.)