Q: My breaks sometimes stop car completely and sometimes it doesn't what would cause this?

Usually a person describes such an experience as more force than usual is needed to stop the car. I am not exactly sure what you are experiencing here, so I will address a few things. Speed and weight of your vehicle will make braking less effective. All things being equal, if you are coming to a stop at the exact same speed and the exact same distance, your master cylinder could certainly be the issue. So could your anti-lock brake system. Your master cylinder can have internal leaks that will affect its ability to create enough pressure at your wheels. Your anti-lock breaks have the ability to do the same thing, but in much different ways.

It sounds as if someone has done a lot of work to your car already without experiencing your issue. This suggests you have an intermittent problem. Intermittent problems are inherently difficult to catch. In order to catch your problem, you will need to leave your car with someone for several days so they car do the best they can to recreate the problem. If stopping is still safe, I would recommend waiting until the problem becomes worse and is much easier to identify.

Usually if it is a anti-lock brake problem, there will be a anti-lock brake light on or the brake pedal will bounce when braking.

I recommend having a certified technician, like one from YourMechanic, diagnose your brake system firsthand to help you figure this out.

Good luck!