In alt.home.repair, on Wed, 19 Dec 2018 00:36:04 -0600, dpb
wrote:

On 12/18/2018 5:20 PM, wrote:
...

I am thinking one of two possibilities. Either the pump on the furnace
does not have enough pressure to lift the water to that second level, or
else that radistor needs some sort of restriction on it's OUTPUT side to
force the water to the top of the radiator. Currently he just has both
pipes going to the radiator, one to the bottom left, the other to the
bottom right.

(This is a HOT WATER, -NOT- a Steam heat setup).

Yeah, if he's running a single-pipe system, the inlet should be the high
entry point and the exit the low for each radiator...

https://www.diydata.com/planning/central_heating/pipework.php#single

Good web page. So since the OP says "he just has both
pipes going to the radiator, one to the bottom left, the other to the
bottom right.", he seems likely to have the second kind, Feed and Return Pipes.