Dual boiler coofee machine is leaking and over heating

You will need to test for where the power stops & check the fuses on each boiler for continuity. If you find any blown then replace the thermostat on that boiler along with the fuses. The thermostats are different rating for each boiler, don't mix them up. It's highly unusual for both boilers to fail at the same time. Occasionally one of the boiler elements can fail, there are 2 elements on each boiler, check them for continuity. If you have any boiler leaks then replace all the seals on that boiler, the steam one is the one most likely to leak. Buy your own seals using the old ones as samples from someone specialising in O ring supply. There are 5 seals on each boiler. Do not purchase rubber or silicone seals, only use Viton seals, they are high temperature.

Cook essentials has nothing to do with the manufacturing of the cooks 12 cup coffee maker model 780-2200

i had a similar problem and found it was just a gasket on the boiler that wasnt sealing properly.i used JB WELD to repair mine.check all connections and/or gaskets for signs of leaks or possibly a broken gasket mating surface.

Three thing can cause this problem.
First is as you mention the air valve is leaking and is only due to its 'o' ring which can be replaced or replace the whole air valve. Not any steam should be leak from it at all.
Second during winter the water supply is much colder than summer so every boiler refill then boiler heating automaticaly switch on.
Third also during winter the room temperature is lower so the boiler is cooling down faster than summer since their in not any insulation around it.
If the presurestat which control the temperature inside the boiler become more sensitive because of years of work this is not actualy a problem since is use the same energy but now in more times and more frequent but for less time each time. Unless it stuck and either you have continues heating with no stop or stuck at alway off so the machine is always cool no need to replace pressurestat.

This is a result of the water temperature in the boiler increasing, thereby expelling water that may remain in the lines out of their respective exit points. A solution may be to place a container under the group head to catch the water, or opening the steam valve partially to allow the water to exit through the steam wand (again placing a vessel/cup/pitcher there to catch the water). This is a problem that even higher end (dual-boiler) models have, due to the necessary purging of water that accumulates in the steam wand during down-time.
Don't look at it as a problem, but simply as a quirk that single-boiler machines have. If you are still irked by the issue, consider buying an inexpensive manual frother (Bodum makes these) to froth and heat your milk (in the microwave after frothing) separately. An added plus is that you can do this while you make your espresso, so if you make primarily milk-based drinks (latte, capps etc.) you'll save on time.

HI
Decalcifying your boiler: Your boiler uses water that has dissolved minerals in it. Even if it is filtered, there are minerals. Your boiler needs to be decalcified to prevent mineral deposits from damaging it. Why can this damage your boiler? The boilers within these machines are small and narrow and therefore they heat up quickly. They hold a small amount of water (250 ml). Their heating element is in the boiler housing itself. So when you start to accumulate mineral deposit, you take up space. This lowers efficiency for heating. Also the mineral starts to attack the silicone gasket that seals the boiler and soon you have a leaking boiler. Water hardness is measured in numbers from 1 to 4. Number 1 indicates soft water having a few minerals while Number 4 describes water containing the most minerals. Set your water hardness to 4 is my recommendation to keep your boiler in good working order. Descale more often than not enough!
To program the water hardness into your machine, press the scoop coffee button and hold until you get the menu. Scroll through the menu until you come to water hardness. Set your water hardness to the level that your test strip says. Press enter (scoop button). Use the arrows to select the hardness and then press enter. You have programmed your hardness and now the machine will tell you when to descale.
Auto descale: To automatically descale your machine, press the scoop button and hold it down until you enter the menu. Use the arrows on the left to scroll down until you come to Descale. Press enter and the machine will enter the mode to descale your machine. The process takes about 45 minutes. You don’t have to do a thing except mix a full water tank with descaling solution and have a container under the steam wand when your start. If you have a cappuccinonatori on your machine you will need two containers. To start the descaling process, simply turn the steam knob. When the process is over your machine will say “Descale finished”. Remove your water tank and rinse it out. Refill it with fresh water. Replace on the machine and start the descaling process over again to rinse your machine. Go back into your menu and scroll up or down 1 to signal descale and enter. Choose “Yes” and push enter so the machine will tell you when you need to descale again.

On your machine, the leakage is the seals on one of your 2 boilers. One can be fixed the other will have to be replaced depending on which one is leaking.

Yes, replace the gasket and see to it that it doesn't
leak.

It is possible that the o-rings in the hose fittings are worn out and need to be replaced.

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Some leaking from the group head of the machine is totally normal while the machine is heating. The reason for this is that the valve that sits above the shower screen is simply a spring and rubber ball. The spring expands, creating a seal with the rubber ball and the boiler, once the boiler reaches a specific temperature. Until the boiler temp is reached, leaking from the group head will occur.