Rear brake master cylinder disassembly KZ650D1

After four ebay and other vendor purchases, I have the parts I need to rebuild the rear brake master cylinder on my 1978 KZ650D1. While waiting for the parts to arrive, I want to clean the cylinder body. Any advice on how to remove the reservoir from the top of the cylinder? There's quite a bit of detritus I'd like to remove.

Thanks!

It's not designed to be removed, so if it's not leaking I would not try to remove it. You may be able to flush it out with lean brake fluid to clean it up and if you can maybe use a small flux brush to loosen any stubborn gunk. Ed