This is an older safety valve that is totally seized up. It is dangerous in that it cannot do its job in releasing excess pressure from inside the boiler.
Use a Stanley knife to cut away the old rubber washer (at the top) and the fibre washer lower down. The rubber just disintegrated to powder on this one. Soak it all in WD40 for half an hour and then work the mechanism to free it up.
Now clean up the brass. Squirt some Cillit Bang on it and leave for a while. Maybe use a bit of wire wool to clean off the tarnished brass.
Once it's clean and free-moving, fit new washers. The rubber one at the top needs to be stretched over the cap. Use a little washing up liquid as a lubricant, if needed. NOTE: The picture shows the valve fitted to a holder. This is not necessary as it is easy enough to fit the washer without.
