Plumbing roof vents, modern vs. old style

[bucolic_frolic]

17. Thanks this is a stumper

To my memory, it didn't work this way when new. It acts like it's not getting enough air. Yet the condition will improve slightly after mass flushing of hot water. As noted, the roof vent is only from the top of the system, there is no roof to basement dedicated vent.

I'm going with a minor clog in the drain pipe, the roof vent pipe, or one of these air-bleed valves in the wall malfunctioning (but it was built in the mid-70s, did they have air-bleed valves back then?

It does have a full S trap on this sink. I'm thinking first step is a P trap replacement.

The drop in the toilets is nothing to write home about either.