How to Bleed baseboard radiators / tankless gas system

jbeebe

I just sweat in some new fittings on the line for my baseboard radiators and there's still a bunch of air in the lines gurgling when the system is on. There is only one bleed valve and it's on the 2nd floor line. I've bled it a couple of times and it just isn't getting out the quantity of air required. I've also tried (referencing the 2nd picture here) closing both shut-off valves and opening the red handled spigot on the lower right of the unit as the system is running, thinking that this is the return line and it will push the air out eventually. several minutes of that and still no better.

So what's the procedure to bleed the lines? Do I just open the red-handled valve on the bottom right of the unit (I think that's the return side) and let it flow out while the system is "on"?

Thanks for all your help!

Here are pictures of my setup.