2 photos show the main valve without the cap. If looking at the white part, there is an O ring on it. You can see the threads are messed up. I guess over the years they became weak. The cap, when screwed down, seals with the O ring and does not allow air to leak out. The pic with the cap shows that it will not thread on right. Once you get it started, trying to tighten causes the side not to go down, thus causing that side to allow air to come out.
The cap has a button that pushed in and out when air is present. It has a very small O ring in it. This button raises the sewing machine needle.