When you press the brake pedeal, does it go all the way to the floor? Have you checked the brake fluid level in the reservoir under the hood?
You generally should be able to replace the wear parts of brake systems (pads, shoes, rotors, drums) without drama. However, if you broke the seal on the hydraulic system (replaced front calipers, rear calipers/wheel cylinders, or brake lines anywhere in the system), you must bleed all the air from the brake lines.
Did you bleed the system? If not, it could cause the behavior you described. It's pretty easy with basic tools & a helper. There are ways to do by yourself as well, but it takes some extra gear.
Go to YouTube and search on 'bleed brakes'. There are lots of videos to choose from. The process is pretty standard among car brands and models. It will vary slightly if you are doing drum brakes vs. disc.
Hope that helps you.
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