How do I get my brakes back to stiff?

Because this car has ABS brakes you may have air stuck inside the ABS controller in which case the valve will need to be opened while bleeding. Please go over this video to show you what I am talking about:

https://www.youtube.com/watch?v=nQRnKyVyMuQ&t=449s

Also, it could be the rear brake shoes are not assembled or adjusted correctly. Here is a guide that goes over the reasons you can have a low brake pedal:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Here is the correct way to adjust and replace the rear brake shoes. Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.