Might the system seal on the valve stem keep the valve in place and prevent it from falling? Brian
It might. But then again, it might not.
It will not because the cam lobe will strike the cam bucket across the gap caused by the partialy open valve.
The valves will all end up in the fully open position with pneumatic failure but long before that the engine will grenade,
unless there is an alternative spring or mechanism to keep the valve components in working contact, this is inevitable.
I can see no need for pneumatic valve closing with the rpm being used.