Spongy Brake Pedal 67 Camaro

[j1m68]

I have to bleed my four piston calipers every two months to burp the air out of them. There is no evidence of brake fluid leaking anywhere. They were stainless steel sleeved 20 years ago and built with the factory style lip seals. Ten years ago I rebuilt them with the O ring kits because they were leaking due to lack of continuous use. Didn't have a problem until 3 years ago when air was getting into them and I had to bleed every few months. I rebuild the calipers again last year and rebuilt the master cylinder this year(that was sleeved 25 years ago). I had the rotors turned so I know those are good. I also have a 68 Camaro with the same factory calipers and O rings installed in those and the brakes are perfect. When I rebuilt the calipers last year the sleeves did have some shiny areas on them. Is it possible that the sleeves are compromised and possibly worn where I should get them replaced with new sleeves. So frustrating to have to bleed my front calipers every 2 months.
Any advise would be appreciated. Jim