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It is the heater control valve or it's connecting hose. Most likely the control valve as it is now about 25 years old. This is on the drivers side of the hearter box above the vent elbow you show in your photo. These are a b***h to replace. It will be a big help if you remove that elbow in the photo first. I took my dash out for other reasoons and replaced my old control vaalve while I had the dash out simply because it was old, and it was easy to replace it with the dash out. Oddly, my old valve was still working OK but the new valve I got from IAP was defective. I had to replace it a second time, but IAP replaced the defective valve free of charge. Attached photos shows the location of the valve with the dash out. With the dash in place, you have to use your imagination.

If you look to the right of the steering column support you can just see the control cable and one of the 2 valve mount bolt heads.
 

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When my heater blew boiling hot antifreeze all over my feet, to get home I just bypassed it (luckily had a length of hose in the trunk!)
I've replaced the heater valve twice now (first new one was faulty....started leaking out between the body where those rivets are, within about 3 months)...no fun.
Did it w/o removing anything but the hard plastic bit that holds the vent hose to the heater body.
Worst part is trying to get the hard steel cable back thru that little eyehole on the heater valve arm with one hand and working the heater cable with the other to push it thru when I thought it was lined up.
A second person to manoevre the hose back and forth thru the firewall (& lubricate this grommet!) when you are ready for it, is v. handy.

take the seat out to lie on your back.....a mirror is also useful, so you don't always have to get your head under there.
 

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I've replaced the heater valve twice now (first new one was faulty....started leaking out between the body where those rivets are, within about 3 months)...no fun.

These heater valves are complete garbage. No one knows where they are made as the Alfa parts stores buy them wholesale. In the Duetto/60s Alfas they are made of metal and have a replaceable inner rubber diaphragm. When they switched to a plastic housing in the 70s, they replaced the four screws with to rivets, and made the diaphragm itself unavailable/un-replaceable.

I've opened one up before and I found that the rubber is always the failure point due to how its constructed. Yours probably had a tear in the rubber along the edge, allowing fluid to leak out. For people whose valve is leaking from the valve shaft, its due to the center of diaphragm being torn.

If anyone knows a guy in the rubber industry - a mold of the diaphragm could be made and produced, with the rivets replaced with nuts and bolts. Then, the parts would cost $5 instead of $75-$100.
 

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For sure a failed heater control valve. The 2 nuts on the base are all that hold it on. There is a moulded gasket between the base and the heater core. The new valve will come with a new gasket. Take the hose connection loose and remove the 2 nuts. Take it off with the control cable still hooked up. Once it is off but still on the control cable, you can rotate the valve so as to be able to see the cable hook up. Make a mental note of where the cable attaches and how much inner wire is showing past the cable stop. If you get the relationship right, you can hook up the new valve on the control cable exactly the same way, before rotating the valve to it's correct position to re atttach the new valve to the heater core. After you get the valve secured to the heater core, play around with the control cable lever to make sure it is moving through it's range of motion properly before hooking up the hose and re-watering the system with anti freeze.

Chill is correct. Replace the hose at the same time you replace the valve. It is a shaped hose, so order a new hose at the same time you order the new valve.
 
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