I'm doing my homework now, I need to now how much air pressure is required to hold up the valve while replacing the springs. My concern is that too much pressure may roll the crank off tdc, and obviously not enough, and the valve will fall.

 

How about none. Bring the piston to TDC and the valves can only fall a small amount and you can get the springs without fussing with air.

 

ok...sweet, thanks