You get a small screwdriver or pick against the inside of each button and push it to the outside of the impact as you pull it outward. The button has a small tab on it that stops it from just popping out. It's easier on the 2115,2135 than the 2112, 2131, because of the lip around the button to catch with the screwdriver.
Once you get them out the direction valve comes out through the bottom of the handle. There's two o-rings on that.
Before you do remove the buttons,pay close attention to the position of the direction valve ( look at in inside the housing, since you say you have it apart) with the reverse button pushed in, as this is the position it will need to be in when you put the forward button back on FIRST, this will make it even as it should be.