Devices and methods for enhancing the operations of fluid systems are provided. For example, this document provides pressure differential relief valves that are well suited for use with medical fluid reservoirs. The pressure differential relief valves provided herein are described in the context of a medical fluid system, such as an extracorporeal blood flow circuit, but the devices and methods provided herein can be implemented in other types of fluid systems including, but not limited to, pneumatic systems, hydraulic systems, fluid power systems, petroleum systems, and various other types of gaseous or liquid-based fluid systems.