Wheel cylinder keeps popping out?

Thanks for the image. I would replace the rear brake shoes and drums; this happens because they are out of spec.

Here are the shoes for $49.00:

https://amzn.to/3KP7cfs

And the drums for $59.00:

https://amzn.to/45A3aj5

Here is a generic guide to help you step by step with instructions in the diagrams below to show you how on your car:

https://www.2carpros.com/articles/how-to-replace-rear-brake-shoes-and-drums

Check out the images (below). Please let us know if you need anything else to get the problem fixed.

You may have the incorrect wheel cylinder installed? If the shoes and drums are new this is the only thing I can think of.