A relief valve assembly to relieve overpressure of a pressurized fluid. In accordance with some embodiments, the assembly comprises a housing with an inlet and an outlet to form a conduit for a pressurized fluid. A normally closed valve member is axially displaceable within the housing to engage a valve seat to prevent a flow of the pressurized fluid along the conduit. A biasing member applies a bias force to the valve member to retain the valve member against the valve seat. A piston within the valve member contactingly engages the pressurized fluid. A mechanically collapsible member resists axial movement of the piston responsive to said contacting engagement of the pressurized fluid. The piston and collapsible member decouple the bias force supplied by the biasing spring member upon the valve member from a force upon the valve member applied by the pressurized fluid, thereby maintaining a bubble-tight seal.