Are you referring to the one that supposed holds the valve open or closed? for use during bleeding? I could never find it but have not had a problem bleeding without it. On my '77 CJ-7 the little button needs to be held in when bleeding so I used a C-clamp on it once but it made no difference. I think on later p-valves the valve needed to be held out. You could probably rig a little c-clip type thing to do this. There are pics in the Chilton's manual I believe, that would be helpful. HTH.
I made one out of a piece of tin, bent to wrap around the valve and hold itself in place, and then with a crease on the flat side to depress the button. Forgot to take it off one time and drove 65 mi. without loosing it!
Got those part numbers out of Chilton's and Hayne's. Took it as far as I could. Nobody has them and manufacturers quit making them years ago. Might find one in in an older auto parts store with an inch of dust on it.
Just bleed them in a normal manner,,,left rear around to right front. I've bleed with and with out the valve,,,never noticed any difference. In fact I should have that tool around here someplace...If you really want it, I'll send it to you. If I can ever find it...