Below is a picture of a vacuum piston for a GL1000.
In the circled area are two punched indentations. These were intentionally made by a PO to identify the pistons placement into the #2 carb body. Nearly visible are two similar punched places in the lower ear of the vacuum cap.
Both punches in the picture of the piston were made from the back side, so the side shown looks like a small volcano. What has happened in this process causes the thin material to crack allowing an air leak.
The punches in the cap have raised the mating surface where it has been punched. This can also cause air to enter but mostly it can cause the cap to fit to the body incorrectly. In this case the piston was sticking because of the distorted cap.
I.D. of these items are important when taken apart, but we scratch a number onto the bottom side of the piston and the mating cap has one on the inside bottom.
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