The brake pedal slowly sinks to the floor when I push to brake. It doesn't happen everytime I brake. Most times the brakes work fine. The other times I'll hit the brake, and the the pedal loses pressure, and slowly sinks to the floor. The brakes are engaged the entire time, but I have to keep pressing the pedal harder in order to stop or to keep the car from rolling once I'm already stopped. If I let go of the brake pedal and press on it again, it'll sometimes work as it should, but most of the time it'll still slowly sink to the floor again. I have no idea what the problem can be. Hoping it's something simple.
Disk brakes.

sticking calipers WONT cause a low pedal. drum brakes out of adjustment or air in the system will cause a "low" pedal. if your car is running and stationary and you push on the pedal and hold it and it sinks the problem is most likely in your master cylinder. it is bypassing and sometimes with this prob. you can see brake fluid on your brake boost depending on which seal is leaking. also you might want to fix it asap not only for safety reasons but if brake fluid gets in your brake boost it will damage that as well. also follow all of your brake lines to make sure they are not leaking. also check your drum brake wheel cylinders and caliper pistons to make sure the problem isn't there either. because if either one of those are leaking it will cause a sink in the pedal as well.

Air in the lines, or, most likely, a Master Cylinder bypass or leak.

First check all the caliper slides for sticking, as a frozen one can cause a low pedal. If it feels like a pedal when you stomp on it but if you just gently press it the pedal sinks its a classic master cylinder seal failure.

theryes a problem with your master brake cylinder