air in brakes?

[Pimpin Dsmstyle]

My question is this; Why can i push my brakes down half way before they actually work? Here is the scenerio : I broke my passenger side brake ( a pad busted off) and i went to brake equip / supply and picked up a new rotor and new caliper loaded with pads for 74 dollars. I removed the other broken brake setup and installed this new one. I bled just that one brake. Later i refilled my res. for my brake fluid. The little rubber black thing under the cap fell off and i just threw it back on. Later i realized that there is a certain way to put it on. So i put it on right and it didn't do anything. Ideas or suggestions anyone*? This is a 1990 plymouth laser FWD turbo make is Feb. ( small brakes, still disk)