drain fluid and remove from bike.Unbolt pivot pin and remove lever.
Poke a 3mm hard rod up the port that carries the hose outlet, over a tray, and with the body supported (above vice jaws?) tap the rod, and the piston, then the seals, springs and the destructable crash washer willl fall out.
After cleaning everything up and making sure that the tiny internal holes are clear, plus (maybe) hone the bore install a seal kit which will include a new crush washer, it is realy the bolted in lever that stops everything falling out, but the crush washer does the job as well, reinstall lever
refit to bike and bleed new fluid through out the entire system, use a vacuum pump, about ?60 from Frost restoration and well worth the money.
CLEM
ps be carefull with the bore size, 15mm and 16mm are common, Brembo sizes, but you could have something else, dont buy the parts untill you have dismantled, so as to be sure, while you wait for parts, you might want to get the bore hones at the local engineering shop, should only cost about a fiver