Main vent "breathing" and water hammer

I've been doing some maintenance on a single pipe steam system and have run into a situation that I'm not sure is expected or not. I was fairly confident that the main vent was the original and was in needed of repair - but also probably not sized for a gas boiler (picture attached). I replaced it with a Gorton No.1 and noticed immediate improvement (less hissing from rad vents, etc). However, I was left with a much louder water hammer than before. My theory on this was that there was a good chance the main return no longer had enough slope. Greater venting + not enough slope = greater water hammer (plus I believe the house has settled in this area).

The thing I'm not sure about is that the main vent tends to "breath" - by "breath" I mean that I hear and feel steam getting pushed through the vent, but it also seems to suck air back in, almost like it exhales slowly and then inhales suddenly. Is this normal?

I noticed it because my system actually has 2 loops - 1 that serves the back of the house and 1 that serves the front. Before doing anything else I decided to replace the other main vent (took some hunting as it is behind a wall in the crawlspace) with a Gorton No.1 as well. This vent doesn't seem to make the same "breathing" noise (or it's possible I just haven't been able to hear it yet).

Is this normal behavior? Or is there a problem on the rear loop where something is causing the steam to retreat and the vent to inhale (vacuum, leak, near boiler piping, etc)? I did have a pro come out before I replaced the 2nd vent. He said the rear main didn't have as much slope as he would like, but seemed to think it was enough. But it was a warmer day and the pipes didn't water hammer like they do when it gets colder (I'm in NC - so cold is relative and there aren't many people that know steam systems around here).

As always - help and wisdom greatly appreciated.