The Bendix® PR-4™ pressure protection valve is a normally closed, pressure control valve which can be referred to as a non‑exhausting sequence valve. These valves are used in many different applications. An example would be in an air brake system to protect one reservoir, or reservoir system from another, by closing automatically at a preset pressure should a reservoir system failure occur. The valves can also be used to delay filling of auxiliary reservoirs to ensure a quick build‑up of brake system pressure. The closing pressure of the PR‑4 valve has a fixed setting.