I ended up hand filing the post in the reservoir but it worked out well.

Then I put the old crappy diaphragm in and let it sit overnight with brake fluid in it.

No disasters, the JB Weld is exactly the same (except cleaner) and no bits coming off or anything, so I reckon that's success.

Then I broke out the wire wheel to finish cleaning the stubborn paint off, and again success.

I made up a frame that I can sit on the workbench in the shed and spray all the brake components in one go.

So if everything goes to plan this week and I can finish the prep of the parts (still a bit of sanding to do), then I should be able to spray on Sunday if it's not pouring with rain.