Hello Vladi. Over the years, the manufacturer of your Spyder changed the design of the rear brake mechanism a few times. Your 2008 is the first design. You do not need to turn the piston as some have said here. You push it in as you have been doing.
So first - the parking brake is only mechanical. It does not use the fluid part of the brake mechanism. When the rider pushes on the parking brake pedal, that causes that cable in the back to pull in. There is a lever arm that the cable attaches to that you do not show on your picture. The cable pulls the lever arm forward. That rotates the shaft. The shaft rotating simply pushes the piston out against the brake pads which squeeze against the brake disk and apply the brake. When the parking brake is released, that spring in the back pulls the lever arm backwards. That pulls the piston back in a little and releases pressure on the disc.
The parking brake does not self adjust. If the parking brake does not get tight enough, you adjust the cable with the two adjusting nuts that hold the cable in the bracket, or you make a larger adjustment by taking the lever arm off and putting it in a different position so that the shaft rotates further. You just try it until you get it right.
Right now, we don't know the condition of your brake pads either, and we don't know if you have that lever arm because you don't show that in your picture. That might be how the owner disabled the system by removing that arm. Your brake pads might be worn and need replaced, also.
You probably did not need to remove that big nut and remove the piston. But, it should not be a problem as long as it has been put together correctly. Be sure to bleed that rear brake to get all of the air out after you are done with everything.
Maybe looking at this YouTube video will help you. It is about replacing the brake pads on your style of brake. It shows all of the parts you should have, including the arm, and how it all goes apart and back together.
https://www.youtube.com/watch?v=NnKVCGc4cLw&feature=emb_logo