So my brakes are weak, as in it's difficult to stop from just rolling at 5mph. The brake pedal doesn't just drop to the floor, but it consistantly pushes fairly easy down, and pumping it doesn't change how it feels at all, no matter how fast, slow, or how many times you pump it.

I tried clamping off the flex lines to isolate the problem, but it didn't change how the pedal feels.

I don't see any fluid leaking anywhere externally.

I thought about changing the brake fluid, but I have a feeling that it's either the brake booster, or the master cylinder. I'm not sure how to know if it's one or the other though.

What do you guys think?

 

Step one.

Bleed the brakes.

 

Ok, will do.

 

Better yet, flush the brake system so you have all new fluid. Odds are it's never been done.
Also, if they've never been replaced the flex lines at the axles are probably way past their replacement time. They swell inside over time and give you spongy brakes.