old cast iron radiator fills w water tho vents new and tilt is good and rads on floor below r fine

It's not the vent's fault. It's not the tilt. It's nothing to do with pressure or vacuum in case that comes up. It's highly unlikely to be something in the fins (wait, fins? What kind of radiator is this? Is it a convector? Photos would help)

I would suspect that even though you think the valve is fully open, it has some kind of problem that is making it be partially closed (like the valve seat having rusted away from the stem and lying in the bottom of the valve.

The other possibility is ridiculous carryover driving water all the way up to that radiator. Even though you said no hammering, strange things can happen—check the gauge glass during boiler firing.

I don't believe in such an event as a pocket of water in a runout, but my beliefs can change over time and strange stuff does happen.

But a radiator runout pipe is too large to hold water the way a straw does in that famous glass of water experiment.

Unless what you are saying is another way to say what I said: "The other possibility is ridiculous carryover driving water all the way up to that radiator."

although thinking more, if the runout is relatively small, too small for the combined edr of the radiators, the velocity of the steam could be pulling the condensate with it and collecting it in the first radiator.

The runout should only be small (and 1" at a minimum) only if the radiator is small. If the radiator is small, the amount of condensate is tiny. The amount of steam flowing into a single small radiator does not provide enough velocity to pull condensate anywhere.

Of course, that's provided it was designed correctly and not modified in some insane way.

@teddyj,. see this thread:

and the video on differential shock in it.