Water disappearing - need some help !

[TobyArnot]

WARNING - THIS IS KINDA' LONG, BUT I WANT TO BE THOROUGH.

O.K. - I need some help before I lose what's left of my mind. In the last few weeks, my 350/350 '70 C-10 Stepper has started "drinking" water. I say "drinking" because I am having to add 1/2 - 1 gallon of water to my radiator daily. I and several car-buds have looked all over and we can't find any obvious leak. Nothing dripping anywhere. New 16# cap holds pressure on 1-ton big radiator. No problem overheating (except in a slow drive-thru on a hot southern day w / A/C on. ) LOL!
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Now, I would think "blown head gasket". O.K. No water in the oil, no oil or exhaust in the water. No water in the tranny.

NO RUNS, NO DRIPS, NO ERRORS !

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I have put nearly 5000 miles on this truck with no significant problems. Still runs fine with no high speed miss. Buddy followed me for several miles & no sign of white smoke out exhaust.
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This is what I get for spending too much money on her.
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DETAILS:Basically stock 350/350 - PO likely 25-30K miles??
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Only other thought I've had is a hairline crack in a cylinder head between the water jacket and the exhaust port. "Sucking water out at speed???" I really don't want to tear into a good running engine, but this has got to stop.
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O.K. gang, Help me solve this puzzle. It's my Daily Driver.