Master Cylinder Piston? Disc Pads? Front Forks?

I am rebuilding a 1980 KZ550 that sat for 12 years, it has 15,000 km on the speedo. When I opened the master cylinder it was filled with gunk (see pictures) so I cleaned the reservoir but I can't figure out how to get the piston stop ring out (see diagram) it doesn't appear to be a snap ring with loops. I want to get the piston out so that I can check the rubber cups and clean out any gunk. Can anyone help me?
I also took the front brake caliper apart to clean, the disc pads still have 3 mm (inner) 4mm (outer) of wear the manual says replace at 1 mm. Do you think I am ok continuing to use the old pads?
The forks after i cleaned them up look ok the dust covers and seals in good shape no leaks and the forks compress ok and rebound good. I only plan on changing the oil is this ok?
Thanks
Tomas

The piston was only held in by gunk, I soaked it overnight then I used a chopstick from the outlet and pushed gently and the piston came out. The white plastic liner was the only thing holding the piston in.

Now on to replacing oil in the forks.

Tomas