Can a heavy piston rob power?

[TonyL]

Inertia is a property of a stationary mass, Jon. A moving mass like a piston has momentum, and too light a piston reduces momentum.

To really split hairs, inertia is a property of a moving mass only if that movement is uniform, which the piston is not - it's either accelerating or decelerating (apart from an instant before it bounces).

The important point is that it is the reduced momentum of the lighter weight piston that makes it less able to overcome opposition from the compressed air.

Great to see you on, Jim.