Proportioning Valve

[GSD Guy]

Hey everyone, I'm getting ready to swap out the MC on my 72 Blazer because the seals are going and wanted to get my ducks in a row before it's not capable of stopping. I know that the button on the front of the proportioning valve needs to be pressed during bleeding but when I took the rubber boot off of the button I was surprised to find that it already seems depressed. There is a small (about 1/8" diameter) green plastic "button" sitting inside of a round raised metal stud (about 1/2" diameter). The green "button" is sitting flush against the metal stud and can be depressed about 1/8" (therefore recessing it into the stud) with the tip of a screwdriver or something similar. This struck me as odd because there is nothing sticking out to "depress" for bleeding, so I'm not sure how the homemade tools would do anything while bleeding. This leads me to think that this is not normal. The truck seems to stop ok, minus the occasional miss by the MC. Any help would be appreciated, Thanks!