Thanks for confirmation on relay id.

I have ordered a new check valve to replace the original(?) one that I cleaned out years ago when I also re-soldered RCM connections to deal with P0410 issues. (Unclear which action resolved problem.)

State of my RCM now: clean, good-looking solder joints; only observable problem was the gunk on the air pump relay's contacts.)

If new check valve doesn't resolve problem, I will replace relay. If that fails, I will email you.