Overflow with problems - Open in Draft

HTML

<div id="container">
    <div id="menu_bar">
        <div id="links">
           links
        </div>
        <div id="full_bar">
           full bar
        </div>
    </div>
    <div id="contents">
        contents
    </div>
</div>

CSS

body {
    padding: 0;
    margin: 0;
    position: relative;
}

#contents  {
    width:950px;
    height: 300px;
    margin: 0px auto;
    padding: 40px 0 0 0;
    background-color: red;
    position: relative;
    z-index: 1;
}

#menu_bar {
    color: white;
    position: absolute;
    top: 0;
    width: 100%;
    height: 40px;
    background-color: blue; 
    z-index: 990;
}
#links {
    background-color: green;
}