Repiping monoflo returns, how does this look?

In theory you could do it with a big pump cart and big connections, but you won't get enough flow with a hose.

I would use ball valves for the replacement of those valves so you can balance the system with them. The gate valves I guess were just isolation valves.

You can use copper for the replacement, it doesn't have to be black iron, be a lot easier to do all those fittings in copper although black unions and nipples for the final connections to the system will be cheaper than copper unions.

Ball valves are a great choice for replacing those old gate valves. Remember to exercise them a few times a year to keep them usable in the future. If you are using them as balancing valves then mark the spot where the handle is after you get the system balanced. One should be 100% open and the other restricted (partially closed) at the point when both returns get hot water back from the supply within a few seconds of each other. It's not rocket surgery, so don't stress on geting it exact. 15 to 30 seconds from the time loop "A" returns hot from the time when loop "B" returns hot is good enough for government work. So it should be good enough for you.

the monoflo tee just partitions the pipe in to 2 sections, one connects to the main, one connects to the branch.

the dissolving and undissolving air won't work unless you can get enough air out of the emitter for water to flow through the emitter and you won't get that with the flow through a 3/4" garden hose or makeup water supply. you would need enough flow in both the main ad the branch to have enough flow exiting the emitters to pull the air down hill with it.

@sixplex said:"

1. If i let the pressure build up to just under 30psi in the system, then purge a bit and repeat, why wouldn't the air get mixed in with the water move along the system?"

This is a point that @DanHolohan mentions in his story about how the book Pumping away came to be. Dan states that our work is dirty and we want to stay away from the clean white walls and white convector covers upstairs. By pumping away from the expansion tank and raising the pressure, and letting the pump run at the higher pressure to get more air dissolved in the water, you may be able to avoid going upstairs to vent convectors in a MonoFlo® system.  My experience has been mixed.  I needed to vent the convectors on systems that were not piped as instructed in the Pumping Away seminar I attended, and the book I purchased.  But when it is piped just as Dan explained, the system purged the air itself over a 24 to 48 hour period of constant pump operation and cycling the burner only on the thermostat. It only worked on a single floor system.  If the radiator risers went to the second floor, I needed to vent the radiator.  

You need to understand that when purging, you are causing the air bubbles to be pushed by the velocity of the water through the pipes in order to force the air bubbles to flow downward against the force of gravity.  When using Pumping away to remove air, the air actually gets dissolved in the water due to the higher pressure and lower temperature of the water.  This is Boyle's Law of dissolved gasses.  Think of it like relative humidity in reverse.  The air molecules actually dissolve in the water, just like water vapor is dissolved in the air all around us.  Temperature and pressure make the amount of air that water can hold change. Just like temperature and pressure change the amount of water vapor the air can hold.  That water is invisible until the temperature drops below the due point and we can see the water vapor as fog.  You can see the water but you can't fill a glass of water with it.  It just floats there as micro droplets of water.  

The air in the water when pumping away gets absorbed in the radiators and then when the boiler heats the water and the pump inlet’s low pressure acts on that water, the air leave the hot water as micro bubbles. (fog in reverse) But that air will just get redissolved again, unless you have some way to separate those micro bubbles from the water before they get to the pump and get pushed back into the higher pressure of the discharge side of the pump.  That is what the different air scoops and air separators do.  This process is using the physics of water to remove the air and it is not instant.  Purging and venting is instant.   It may take several hours or several days depending on the flow rate and the amount of air you are trying to remove using the pumping away idea.   I can guarantee you, the you will need to vent any radiator that has very long risers from the basement to an upper floor.  Pumping away will take too long to do this center radiator on the second floor.  

The way you drew it is fine. I would put a vent at the end of each monoflow return before the ball valve (vent upstream). You will be able to get the air out of the main. Then venting up at the radiation will be less