The lid of the steam valve can be unscrewed from the main body - the diaphragm should then lift out. I found it necessary to gently heat the body of the valve before using a ring and 'C' spanner to unscrew the two elements.
As can be seen in the picture below the steel 'sealing' stud is completely corroded and would not close off the top of the overflow pipe.
The diaphragm and overflow pipe will be discarded in favour of the later type of ball and spring relief valve. The two vent holes in the lid have been closed off with solder in preparation for the conversion.
|Up-side-down to show the sealing stud - which should be perfectly flat.|