The bounce test fools almost everyone You press a trumpet valve down and let it pop back up, and if it moves freely, you call it fine. Actually, a valve can bounce freely and still be worn past the point of holding air, because the seal that matters happens along the sides of the valve, not at the top where your finger presses. The gap between a Monel valve and its casing is measured in thousandths of an inch. A quick press-and-release tells you almost nothing about that gap.