Impressive. What kind of valves? Including the boiler?

It had a sliding double-action (push-pull) piston and a push-pull valve operated by a linkage attached to the flywheel.

I didn't make a boiler - just ran it on compressed air.

It vibrated ferociously, though, as I had paid no attention to balancing it.

Impressive. A wobbler would be doing the valve on easy mode.