The master cylinder I bought was for a 2007 Silverado 1500 with disc/drum which is what was in the year span mentioned in the first reply, 99-07 and mentioned in various threads. The link provided only has part numbers for the adapter, I didn't see anything for mc part numbers. That thread is the one that suggests to use a residual valve as the masters listed are for disc/disc see page 7 and the posts by knuckle dragger. Also 2005 and up is when they were changed back to drums on the rear at least that is what I read.
As for me, I did read and went to other sites as well. There was no mention of needing a specific mc as any would work as long as it was for vacuum. After finding out that is wrong I searched for a disc/drum mc and all the ones I found used hydro boost or were disc/disc in the 99-06 range. No one was able to provide a part number for what I needed help with which I won't fault anyone on. I spent hours trying to find one in 99-07 range and the one bought and returned was the only one I could find. So I am going with residual valve as the Mc I bought won't work (the m12 one) and I really don't want to keep buying mcs and hope one eventually works.
Thanks for the help though everyone. Will report back how the truck is after the valve install.