Why _y coordinates do not work in my code?

1

Now I have a code that works perfectly but something is wrong with the y coordinates.
The mc I want to tween is a menu button. I just want to change its y coordinate up on rollover and down at its original position on rollout.
my mc has a y coordinate at _y 53 and should go up at _y 43 on rollover
and it should go down at original _y 53 on rollout.
So even if I respect precisely how the code should be built it does not do what I want.
Or the mc button goes to high or it does not move at all
What is wrong again ??? I go mad.
so the code is
import mx.transitions. Tween;
import mx.transitions.easing.*;
menu1_mc.onRollOver= function() {
new tween(menu1_mc,_y,Bounce.easeIn,menu1_mc._y,43,3,true);
};
menu1_mc.onRollOut= function() {
new tween(menu1_mc,_y,Bounce.easeIn,menu1_mc._y,53,3,true);
};