Coming off the radiator. Small leak coming out of the left side of the rubber gasket

There is a leak somewhere in the cooling system but this seems to slow to be the culprit.

What’s the best way to find where the leak is coming from. Takes about two days for the resivoir to empty

 

???
What year, make, model, etc.
Where you located?

That looks like trans line leakage. Probably in cab heater core small leak, and if your in a humid climate, you wont notice it.

 

I don’t see a rubber gasket so I’m not sure what you’re looking at?

Top picture is hard to tell if the side tank has a leak running down from near the top or if it is a shadow.

Bottom picture looks like someone patched together the transmission cooler line with a piece of rubber hose.

 

Air inlet to box should be a gasser V8. I had a slow *** coolant loss for years on my 302, ended up timing cover to block gasket failed. With a truck around 40 years, suspect all unless documented repaired.

Yes, that patch job is comical at best. You need to purchase new lines if available, find clean used, or make some. Advise to add external ATF cooler if not currently equipped

 

Engine degreaser and a power wash will make all leak detection operations much easier. That fluorescent leak detection fluid can also help with or without a UV flashlight.