Is there evidence that dating methods MUST be invalid?

Creationists argue that since C14 has not yet reached its equilibrium rate, the age of the atmosphere must be less than 20,000 years old. It is possible that a greater concentration of water vapor existed prior to the Biblical flood (presumably about 5,000 years ago). This water vapor may have retarded neutron production by cosmic rays and consequently diminished the yield of C14. This would give early fossils a low C14/C12 ratio and therefore the appearance of great antiquity. On the other hand, if a lower concentration of water vapor existed, there may have been a greater amount of C14 produced; the increased C14/C12 ratio in the fossil would be interpreted as a relatively young age. The water-vapor content of the atmosphere has varied considerably in the past, thereby disturbing any C14 equilibrium that may have been attained.