Why did the roof pentamirror in my Maxxum 70 deteriorate with age?

I have a Minolta Maxxum 70 which I love using. The viewfinder is not as sharp or as bright as it once was. I was told the this camera uses a roof pentamirror instead of a pentaprism. I was told that this roof pentamirror was of poor quality and not supplied by Minolta. Can this roof pentamirror be cleaned or replaced? I would love for the viewfinder to be as bright and sharp as it was when I bought the camera.