Decades ago 2 others and I bought a row house unit built back in the late 1800's as a fixer upper. First thing we noticed was the 10 or so minutes it took for the toilet to fill after a flush.
We only worked on the unit part time and when we finally got around to fixing up the plumbing and got the supply line replaced, we hooked the air compressor to the supply line we had removed.
The pipe didn't let any air pass till we had the air pressure well past 40 psi and even then it was only a tiny stream of bubbles.
We all just about passed out taking turns trying to blow into the pipe to get air out the other side