From one hose to the other,
not from the cap to the exit (which is the same header tank)
Chlorine is horrible for brass. Ask anyone who reloads ammo.
So Hydrochloric acid is about the
WORST thing you could use.
There are plenty of products to remove scale and rust.
CLR or Iron Out are going to have sulfamic or citric acid.
Acetic acid (cleaning vinegar) is cheap enough to fill a radiator and easy enough to find.
If your radiator is working, don't go looking for a reason to change it.
But aluminum has close to twice the thermal conductivity of brass.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.