i was driving today down a small incline and a step on the brakes and all of a sudden it lost all it's pressure. it grabbed a little, but the pedal sank to the floor. what could be the problem. i checked the brake fluid and it's filled to the top.

 

its probably your master cylinder man!!!

 

there is no probably, it IS your master cylander... either that or all your break lines suddenly developed holes in them... and i dont think that break boosters go bad

 

replace your master cylinder, put some fresh fluid in it, bleed the system and you're set to go. while you're at it, check your pads and rotors....