I have a '69 SS chevelle with power front discs and rear drums and i was wondering if the button type thing underneath the rubber boot on the disc brake hold off valve is supose to be able to be pushed in and out or not?

Short answer...yes. You push it in when bleeding the brakes. Sometimes they get stuck and need to be taken apart and freed up. I have one on my bench that I just did this very thing to.

is it easy to do? cause a new one is over $100 for the stupid little part