Problematic Instant hot water dispenser...

Hi everyones...thanks a lot to read. I'm trying to repair for my aunt this dispenser that is leaking just under the faucet (only when she draws water). http://www.insinkerator.com/pdf/h770.PDF Of course i could not see exactly where the leak camed from...there's 4 tubes that goes through this small hole in the counter. I suspected the valve in the faucet but i could not be shure. I decided to remove the whole unit including the tank to test it at home in my basement where i have an old sink installed normally for messy cleaning jobs. Once partially diassembled, i tested 1st the faucet itself . No leaks, perfectly working. I then bought a few new compression rings and then tried the whole system connected together. When i started to fill the ½ gallon tank thereafter, i could not see any leaks at all. When that tank was entirely filled and water started to flow from the faucet's spout, i've noticed a few seconds after that the plastic vent tube was starting to fill and eventually reached the faucet's internal vent well and eventually water leaked from under the faucet just like when it was installed on the counter. Please see this page i've scanned from the installation guide: (sorry it's in french, but i've translated the more important parts)
http://www3.sympatico.ca/kilo.watt/images/h770_install_guide.jpg
If i understand correctly, there's an expansion chamber that normally should not contain any water, and this chamber is to allow for the expansion of water when it's getting hot...so the tank is never under pressure in any way. Do i get it right? To separate those two sections, there must be a kind of separator membrane just like in those pressurized tanks (precharged with compressed air on on side, and the water on the other side) used in house that get water from a well via a pump...i'm i right? It is ok to assume that my leak is caused by a bad tank that lets water go in the expansion chamber and right into the vent tube toward the faucet? That's a lot of ? and any usefull replies and comments are welcomed. TIA
-- Alain(alias:Kilowatt) Montréal Québec PS: 1000 excuses for errors or omissions, i'm a "pure" french canadian! :-) Come to visit me at: http://kilowatt.camarades.com (If replying also by e-mail, remove "no spam" from the adress.)
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Kilo, Yes the tank is defective. If it is still under warranty, the call the 800 number for service. (reinstall first as they come to the owners house to fix.) If out of warranty, then replace tank or probably more cost effective, the whole thing. Dale

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Hi Dale. Thanks for your reply. The unit was installed since 1996 so warranty service is not a consideration. We are both probably right about the tank. I've removed it from the casing and it's insulating jacket yesterday:
http://www3.sympatico.ca/kilo.watt/images/h770_tank.jpg
By rotating or tilting it, i can hear that there's a loose metallic-like part inside! Time for her (my aunt) to buy a whole new unit! :-) Thanks again. -- Alain(alias:Kilowatt) Montréal Québec PS: 1000 excuses for errors or omissions, i'm a "pure" french canadian! :-) Come to visit me at: http://kilowatt.camarades.com (If replying also by e-mail, remove "no spam" from the adress.)
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