Rear brake cylinder end cap fell off during brake shoe swap.

[CoryM]

The piston came out and you likely have air in the system. Time to bleed out that corner. It is possible that you have damaged the piston seal, but probably not. After you are done bleeding, have your helper hold steady pressure on the brake pedal as if they were coming down to a hard stop. Make sure you have no fluid coming out, and the pedal height remains the same while doing this pressure check.

With brakes, pretty much any time you see brake fluid come out, you should bleed.
Cheers.