Need input on valve stem seal replacement

[rafterman191]

I have the chuck to keep air pressure on the cylinder through the spark plug hole. How much pressure do I need to keep on it. What exactly is this doing? Other options on how to do it? Come to my house and get greasy tomorrow?

 

[4X4HIGH]

The air is to keep the valves closed and from dropping into the cylinder while you change the seals. How much air is needed i really don't know but it isn't hard to do.