Hi,

I think you'll find that if there is no valve spring pressure the tappet will jack itself wide open under oil pressure and will hold the valve open slightly.

This is exactly the reason you have to squeeze out all the tappets when you remove the cams or followers on any engine with hydraulic tappets, otherwise you can have trouble starting the engine due to lack of compression

When the piston is travelling upwards on it's compression stroke there is no inrush of air due to a higher pressure on the underside of the valve.

Regards: Bart