I have no idea what this is - they scabbed first - then they started popping brown puss?

[awooga]

My San Pedro Collection - it has spread through six large pots of about five in each
Another pic

https://imgur.com/a/ckl7V

The brown ooze turns sap like too after a while - is this common?

 

[Gone-and-Back]

That is not normal at all. That looks like some serious rotting issues to me. What kind of soil do you have them in, and how often do you water them? How long have you had them for?

Is the whole cactus affected by it or is it just a portion of it?

 

[Spanishfly]

That appears to me to be some kind of virulent pathogen - much more than just rot. I would BURN every one affected without delay. Sorry.