That's a very informative thread! Thanks. I thought I flushed out the ports really well with brake cleaner prior to installing the piston assemblies (which I soaked in brake fluid and carefully inserted into the cylinder).
However, this time I'll spray into the ports from both directions to dislodge small contaminants and make sure the residual valve is clean before putting it back in.

The Ram Man's video to disassemble & re-assemble the MC was helpful. He has a special tool to pull out the front piston assembly. I guess I'll make one. Apparently using compressed air to remove it ruins the cup seals.
Thanks again for your help.