I apologize for such a late reply, but you might just want to make sure that the flipper switches that you are using can handle the current. There are two different styles depending on the age and location within the machine. With many earlier pinball machines, the current went through the flipper switches as you have shown. The switch will have much larger contact pads (maybe 1/4 inch in diameter), and the contact pads are normally made of silver alloy. Low current pads are only about 1/32 inch in diameter and have a pad made of copper. If using a switch with a low current pad in a high current application, the switch can arc and "weld" itself together. Doesn't happen very frequently, but it can happen. You can also see a lot more oxidation on the pad because the arc is eating away the metal.

The reason I'm pointing this out is that if adding a pop bumper, you may run into the same sort of a problem. Pop bumpers take a lot less current than flippers so it might not be an issue, but it is something to look out for. Just take a look at the switch pads after a week and see how they are holding up.