Master Cylinder - Can't get old piston out

Hi guys, I'm a newbie on here and to the CB500.

My girfriend and I have recently purchased a 94 CB for a winter bike for me and first bike for her. It had been off the road for a couple of years and I have been ironing out a few wrinkles before putting it in for the MOT. Fork seals replaced, caliper overhaul (picstons & seals) and a new braided hose. Thought everything was hunky dory but after an hours riding around a car park found fluid dripping out of the master cylinder onto the tank and side panel. I bought a service kit and on taking the dust cover off noticed that it looked a bit crusty and brown. After the best part of half an hour with a can of brake cleaner, ear buddies and a bradawl I managed to free up the circlip and get it removed. Fromt he Haynes diagram and the service kit, I anticipated finding a washer next. Can I get theis out....nope. I have tried tapoing with hammer and screwdriver all the way round, heat from a hair dryer combined with a squirt of brake cleaner for contrast, more tapping. I have even tried to grab the end of the piston with pliers and jerk it towards me. As a last resort I got a punch and from the cut out where the lever has been removed, i've attempted to tap the washer round to free it off. The material seems to be really really soft. I was expecting steel, but judging by how its dented, it may be aluminium. I am now stumped as to what else i can try. My next thought was to remove the banjo and assuming its a clear bore through to the piston, to try and drift It out from that end. Any suggestions and observations gratefully received.

Just looked at my Haynes manual and it says/shows what you've said - circlip followed by a washer.

The only suggestion I'd make is that the washer is corroded in place - you might find a Dremel (or similar) with one of the small wire brushes will clean the crud and corrosion out and let you see what's holding the washer in - might even remove it altogether.