1995 Lincoln Mark VIII used Master cylinder

Brakes problem
1995 Lincoln Mark VIII Two Wheel Drive 120, 00 miles

how do I know that a junk yard master cylinder is any better than the one I am takeing out. Dealers tell me that new part is $300 scrap prices are 10 to 100 dollars. They come with a grade if A, B, C, D, etc
What questions do I ask. Can I do this replacement myself