For everyone with bad service buttons:
Three screws (ground strap, and two tiny black metal screws).
Take the unit off, unplug. (Plug is on tighter than you think, but it pulls off).
Use two small flatblade screwdrivers on the clips of the back side, gently pry the green circuit board up a little without deforming the black plastic clips.
Lift the green circuit board up a tiny bit at the connector end by prying with one of your screwdrivers, then insert your screwdriver to the side of the wedge at the end furthest from the connector. Pry the whole circuit board forward until the end clears the black wedge ramp at the end of the circuit board.
Continue to gently work the circuit board upward over the various black plastic clips with the screwdrivers and you'll get the green circuit board out.
Then the four buttons (Green, Red, Red, Black) will fall onto the floor. Pick them up. Hehe.
The circuit board has a translucent button rubber on top of it, gently pull this off, pulling the tiny pegs of translucent rubber out of the circuit board.
Clean the circuit board metal contacts that were under the buttons with 99% alcohol. Don't leave any lint or residue on the circuit board when you are done wiping it off.
Place the translucent button rubber piece back on the board, aligning the tiny rubber pegs. One by one, gently pull the pegs through the board until it the translucent button rubber rests flat on the circuit board.
I plugged the board back into the game at this point, and was amazed at how well the translucent button and green circuit board worked. I didn't have to clean the black surface of the inside of the translucent button cover with alcohol, but if you didn't get good results, now is a good time to pull it apart and clean the circuit board and maybe the black pads again with 99% alcohol.
After verifying that the button circuit board was working perfectly, I unplugged it from the pinball, and it was just a matter of loading the colored plastic buttons (Green, Red, Red, Black) back in the holder and popping the circuit board into the holder.
Plug the button unit back in. Put the assembled button unit back on the front door with the screws.
You now have perfectly working buttons.