Rather than reinventing the wheel, have you thought about looking at an actual ventilator? A quick search for “medical ventilators” on eBay yields a few suitable devices for sensible prices. These ventilators should give you a reliable solution with all the necessary control surfaces to reproduce a realistic effect. If you go down this route you probably want to avoid using anything that needs compressed gas to work. Non-invasive type ventilators meant for home use are probably the most suitable. The effect you are trying to achieve is regularly done in medical simulation for training purposes so should be reasonably straight forward to put together. Your local anaesthetic department may well have a stash of outdated kit/historical ventilators that you could borrow and cobble together. There’s lots of information online about how these ventilators are designed if you particularly want to build a bespoke unit. If you need any advice regarding setting any of this up feel free to make contact, I’m training as an anaesthetist at the moment. So making lungs go up and down is a bit of a special interest...
