ok I changed the pads on all four yires and needed to replace the two rotors on the back tire of my 2000 mountaineer .my truck stops but my pedal is going all the wy to the floor what is the problem a

And it didn't do that before your "brake job " ? If there's rear calipers with the emergency brake they may need to be adjusted , they may not "pump up " to fit right , and like they said , it needs to be bled properly , if still no good bleed the master properly , If unsure about any of this perhaps you should have a shop do your brakes .

Fluid level is your likely answer. Changing pads and rotors will not affect this unless you know you did something to lose brake fluid. Check your fluid level in the reservoir by the master cylinder (under hood, driver's side, at fire wall). I'll bet it shows you need to add some. Be sure to use the right type of fluid if you need some. Probably DOT 3 but it is usually printed on the reservoir cap.