The valve rubber is part of the gas inlet valve, and is a specially shaped rubber gasket that is placed over the valve needle. Its function is to allow gas to flow out of the charger towards the inside of the siphon, but not out of the siphon. It is the second most important part of the siphon head and is subjected to the greatest stress. When the charger is pierced, the gas flowing in under high pressure expands and the whole valve freezes immediately.
Over time, the material of the valve rubber will age and become rigid. This is almost always the case with old siphons. Replacement is recommended at every maintenance. It is best to apply a thin coat of silicone grease to the valve pin and the outer collar of the valve rubber for a better seal.
If the valve rubber fails, the siphon will lose gas, and as the pressure drops, not all the soda water or cream can be extracted. In the case of a soda siphon, water may leak at the cartridge holder.
When the gas inlet valve is removed, the valve rubber can easily stick inside the siphon head and is very difficult to access. The best tool for pulling it out is a crochet hook of suitable thickness. A good thickness is about 4 to 5 mm, if thicker, you may not be able to fit it inside the valve rubber.