I'm working on a drop down menu that is setup in WordPress. (The theme i'm using is a starter theme from http://underscores.me but the CSS is mine)

The issue I'm having is the sub-menu item doesn't hide. I double checked my code and even pulled the menu code from the twenty eleven theme to see if it was an error in my code, but I got the same result.

The construction site is located at: http://www.jcorbindesign.com/wordpressbeta/ and I've provided the CSS below. Any help would be greatly appreciated!

Code:
#menu-main_nav {
    background-image: url('./images/navibar.jpg');
    background-repeat: no-repeat;
    height: 70px;
}

#menu-main_nav li {
    display: block;
    float: left;
    height: 70px;
    line-height: 70px;
    position: relative;
}
     
#menu-main_nav li a {
    display: block;
    height: 70px;
    line-height: 70px;
    padding: 0 15px;
}

#menu-main_nav .current-menu-item a, #menu-main_nav .current_page_item a, #menu-main_nav a:hover {
    color: #939598;
    background: #fff;
}

#menu-main_nav ul ul {
    display: none;
    position: absolute;
    top: 70px;
}

#menu-main_nav ul ul li {
    float: none;
    width: 150px;
}

#menu-main_nav ul ul li a {
    padding: 5px 10px;
}

#main-nav ul li:hover > ul {
    display: block;
}