brake question on my 57

[Groenys 57]

I recently got my truck back from swapping it from a manual to an automatic. The swap took a little longer then I planned on. Long enough that there was not snow before the swap, now there is snow. When moving the truck from the garage to the trailer, I noticed that the back brakes did not work. I figured this out because they were still spinning on the ice when the truck was stopped. The front brakes held the truck in place while the rear tires were still spinning on the ice. I managed to get it to my shop where I jacked up the back and tried to bleed the rear. Fluid was coming out so I figured I was good. With it still jacked up I put it in gear and pushed the bakes. The rear tires were still spinning.
I removed the master cylinder cap from can see fluid in it but I'm not sure how much is supposed to be in there? It also looks very brown and old. I don't think it has been changed on the 15 years I have known the truck.
Should I just try to flush the whole system and start with new fluid and see if that helps? If so how do I go about Flushing all the fluid? How can I see if it is the brakes themselves and not just the fluid?

I know its a lot of questions but I'm new at working on my own vehicles and don't know where to start.