It seems to be leaking where the plastic cylinder attached to the metal base. Anyone ever encountered a situation like this? Any help much appreciated.
These rely on a mechanical seal formed when the plastic part is driven under pressure into the metal part. It is a one-time deal and if the components are ever separated the piece that forms the seal is destroyed and you need a new plastic part. New plastic parts are NLA since about 2004.
My experience is that they all leak after a while due to age related deterioration of the plastic, aided where I live by the generally high UV.
As others have suggested, measure the diameter of the bore and replace with the newer "square" type, which is a direct bolt on to your existing lever/throttle assembly.
Having just looked at the Motobins site, you can still buy the "round" brake master cylinder, but unless you are a stickler for originality I would pass on that and buy a square one as a round one is just as likely to fail on you in the same way as your existing one.
