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I put the cut peaches in the syrup to heat through the fruit then I placed them in the jars and topped off with syrup half inch from top.I had problems at that point the peaches kept wanting to float up. I used wide mouth masons.
I tend to cram the fruit into the jars before I seal them. This eliminates some of the space in the jars. Takes more fruit to fill, but the jars remain full, top to bottom.
When my grandmother did peaches, they floated... when you put a ripe peach in water, it floats... apples float... and I do know a can of beer floats in water... sooo, I'm saying all good things float...
The sugar in the water made it more dense... floating is OK....
If you process properly according to Ball/Mason/Kerr there will always be an air pocket on top. It is although a vacuum area and not just a void. The vacuum makes all the difference.