the brakes go all the way down to the floor after / while riding on the highway... but local road the brakes are fine... i didn't bother bleeding it yet... its too cold outside... i still have warranty wat do u suggest me doing...?

 

.....I'm kinda new here, But what I would do is call the dealership and ask if the inspection and/or repair will be covered. Sounds like the fluid has oxidatized to some extent and when you need to stop from higher speeds (More brake usage) the problem is more apparent.--I don't know maxima related problems, I'm to green-But I would bet a brake line bleed/flush with new fluid will solve your problem.
No vaibration,warped rotors, or really thin pads right???--
I would have other ideas, but they are for drum rears.

 

Possibly blocked (aired, can't be frozen lines) or maybe need bleeding. In city, couple of back to back pumps on the brake pedal must be creating the pressure (I doubt your brake bottle is open leaking). Were brakes recently changed? Color of brake fluid(crystal clear)? Since its in warranty, then have the friendly team of a Nissan dealership go at it...

 

Bleed them Captain Obvious.

 

Kabir I can help you bleed the brakes sometime. Let me know.