Right-front brake slightly dragging

Kendog 170 said
A collapsed flexible brake line can act as a check valve (collapsed inside the hose) and make a caliper drag. I also once installed a 1974 disc set up and had to add a shim between the mc and firewall because both sides were dragging. I loosened the mc bolts enough to diagnosed that that was the issue. I didn't realize there's a gasket. Just spit balling here.
View attachment 1716440972

This might be something, I did replace the MC years ago and I don't recall if I put a gasket in there; kinda hard to tell from the pic but is that gasket compressible (foam) or a harder material?

I won't rule it out but I doubt it's the soft line as I replaced those with SS lines that I optioned with the Dr. Diff kit.