Did you try and bleed the brakes manually to see if there is any air in the lines someway/somehow? I have a master on my floor with anti residual valves as well. Sometimes mine will be stiff then soften up. But I never lost brakes. I also had a stupid anti proportioning valve that I had adjusted wrong years back. But that’s another story. I would personally try and bleed them manually and see if any air is trapped in the system before replacing the M/C. Those wilwood master cylinders ain’t cheap. Just my thoughts though.
Something else that may be possible is one of the anti residual valves are sticking since you replaced them last winter. Wouldn’t be the first time something brand new either is junk out of the box or fails shortly after some usage.