Dull Brakes on V35 G35, can't diagnose
 

Hey all, my current (stock) G35 has almost 300,000km on it, that's 186,000 Miles. For about two years now my G35 has had dull brakes. A few key things to note is that once I fully press the brake pedal, it then goes down a bit afterward. Then, the key thing is when reversing at the start of the day, the brakes are so dull that it is like they don't even work. The pedal is extremely dull, and the brake pads just don't seem to press on the rotor. I cannot seem to diagnose the issues, I thought it may have been the brake fluid but I changed that last year, then I thought it would be the master cylinder but what threw me off is the reverse issue. I heard some people mention Plenum, but I'm not very sure about what that means and what that is, and in turn, what that can do. Please help thanks!

There is no plenum on the brake system, ignore that part.

Sounds like you have water in the brake fluid, start by getting a fluid flush, make sure it's only DOT 3 fluid or a DOT3/4 that's good for both specs.