water heater - pilot light

I've got a 30 year old gas water heater. In the past 5 years I've replaces the "thermocoupler" twice because the pilot light would go out.
It's out again and this time the new thermocoupler didn't work. The pilot will not stay on. Is there something else that I should check .... or did I just get a bad thermocoupler?
Thanks, JT
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This is Turtle.
What you say here can have two causes to not work.
Bad Thermocouple.
Flame of pilot to low or short, stopped up, hitting the middle of thermocouple head, or flame of pilot lite not hitting the top 1/3 of the thermocouple like it is suppose to.
TURTLE
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TURTLE wrote:

You forgot the other five possibilities: The thermocouple seat could have trash in it preventing electrical contact or even causing a short. It may have been cross threaded. The gas valve may be weak or out. They may not be depressing the knob fully. The thermocouple may be faulty.
hvacrmedic
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this is Turtle.
hey i did forget that one the gas valve , thanks , but the others i really did not want to write novel this late at nite.
TURTLE
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I tried another thermocouple, different source/brand. I turn the control to PILOT, hold down the gas button, light the pilot and after 1 to 2 minutes release the gas button. The pilot light goes out!
This water heater ran fine for 25 years, then about 5 years ago it started needing a new thermocouple every 2 years. Now even the new thermocouple won't keep the pilot light on. I know I should buy a new water heater, but it's a real hassle.
Since it's obviously not the thermocouple, what else should I be looking at?
Thanks, JT
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