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The diaphragm in a vacuum-operated cruise control servo, split so it cannot hold the throttle. These are found on older cable-operated systems, where a vacuum servo pulls the throttle rather than the module commanding an electronic one.

Cruise control engages and immediately drops out, or holds speed poorly on a gradient, because the servo cannot maintain its position. It is also a vacuum leak, so the idle may be affected and other vacuum devices weakened — which is the clue that separates it from an electrical fault in the switches. Test with a hand pump at the servo: it should hold vacuum indefinitely and move fully, and one that will not hold has a split diaphragm.