I’ve had good luck drilling a 1/8” hole on thermostats to help rid the system of air.

As for flushing:

If/when you flush….here’s a helpful tip I learned here and can attest that it works. Rig up a npt to barb fitting in the block drain hole. Attach a section of tight fitting hose. As you are flushing/ draining (would be good to wait until you have any caustic stuff out of the system)….give an occasional blast of compressed air through the hose and into the block.

You need to have a place to relieve the pressure. I like to leave the rad petcock slightly open. Having a little shutoff valve somewhere in your drain hose is helpful. But you can just as easily get pinch it off to direct when/where you want to drain.

You’ll be amazed how much crap the air loosens up, long after the water has been flowing clear.

1951 3100