The safety valve on a pressure cooker is a plug made of alloy and a soft rubber gasket. In the event that too much pressure builds up inside the cooker, the alloy melts and allows steam to escape, keeping the pressure cooker from exploding. Once the alloy has melted, you have to replace the valve before you can use the pressure cooker again. Some manufactures suggest that you routinely replace the safety release valve on the cooker every six months.
Examine the valve to be sure replacement is necessary. Check that the metal stem through the lid moves freely, and look for signs of drying and cracking in the gasket. If the stem does not wiggle in its seat and the gasket is no longer supple, replace the valve.
Remove the lid from the pressure cooker pot and turn it upside down.
Loosen the nut holding the alloy valve stem, using an adjustable wrench. The valve is usually located on the lid to the side of the main stem, which is in the centre of the lid.
Remove the nut, the alloy valve and the rubber gasket.
Place the new gasket over the hole on the top side of the lid.
Drop the new valve into the hole and tighten it underneath with the new nut.
Check that the new valve moves freely.
Never leave a pressure cooker unattended, and always follow cooking times and temperatures carefully.