I have an AO Smith, Model GCV 40 100 water heater and the pilot won't stay lit.
Make : AO SmithModel : GCV 40 100
3 weeks ago the pilot wouldn't stay lit. I cleaned off the pilot burner, which had some carbon build up, cleaned air intake, checked for clear ventilation to outside. After I cleaned off pilot burner the pilot worked up until last night.
Have you tried replacing the thermocouple? (the copper lead that gets enveloped by the pilot flame.) you can get them at any hardware store. Take the one you have with you so they can match it up with the right length.
Manufacture Rep for Major Brand, Technical Trainer, NATE