1/4" Anti-Vacuum Valve Stainless Steel

There are many names for this part, but a brief description should suffice: this valve allows air to escape the boiler as the water heats up, hissing and then closing off when the boiler water reaches its operating temperature, sealed by the steam pressure created. 

Cross-references: 

Part number	Manufacturer
F.2657	Ascaso
418911010	Casadio
418941010	Casadio
418911010	Cimbali
418941010	Cimbali
537040300	Cimbali
418911010	Faema
418941010	Faema
537040300	Faema
1523073	LF