Main Drain Airlock ?

[Galaxie 500]

First time closing yesterday, seems like it all went to plan. We have a very simple set up, one skimmer with its own line to the pump and then two main drain ports in the pool but they come together with a single pipe back to the pump. A single control valve to select between drains and skimmer.
Questioning myself on blowing the drains to airlock. I used a cyclone blower. For the first ~30 seconds both drain ports bubbled profusely, but then one would stop (I think the further away) while the other kept bubbling. We kept the blower on for another good amount of time, then set the valve to skimmer only to lock.

Is what we saw with the drains to be expected?
We did this step 3 times out of an abundance of caution and every time it acted the same.

 

[ajw22]

The frost line in MO is about 15 inches.

You only need to get water out of the pipes and lock in air down to about 15 inches.

Your pool water will never freeze down to the level of the main drains.

 

[Newdude]

I used a cyclone blower. For the first ~30 seconds both drain ports bubbled profusely, but then one would stop (I think the further away) while the other kept bubbling.

That's fine. If the two halves of the T aren't perfectly symmetrical and level, the air will favor one side over the other. It might even change its mind halfway through. It's all about the path of least resistance. That horizontal pipe under the pool filled back up as soon as you stopped pressure, but it's deep so it doesn't matter.

Wherever the pipe turns upward, has air in that can't glug glug out sideways, and the rest of the pipe to the pad is empty.