Something tells me I was right to suspect the master cylinder was bad.
I have never seen as master cylinder this nasty. Fortunately after some cleaning, the seal & shaft on the booster were still intact, so I shouldn’t need to replace that.
I bled the master, but need to pick up more brake fluid to finish bleeding the lines. Hopefully this will be the end of the brake saga.
The buyer showed up today to pick up the donor wagon. He drove 7 hours each way to get it, and was happy with it. I was happy to make back $500 of the $600 I paid for it. So it was a win for both of us