black water holding tank valve opens but won't drain

1 flush for #1, 2 flushes for #2 with plenty of water (read almost full) each time w/ the first of the 2 w/ the "dirty" contents.

May sound silly and thought my FIL was nuts, but after we dump the tanks, we add 10 gallons to the black tank before leaving along w/ the GEO method. Tank reading isn't worth a darn, but I know tank is drained. Shoot, we went a whole week on vaykay before dumping. Granted the black tank was getting pretty darn close to full, but she drained well.