compressed air method to change seals

I have used air pressure many, many times. It is ideal to have piston at TDC in case you somehow knock the valve hard enough to break air seal, so valve won't drop. Not always possible to keep piston at TDC, like another member stated, but if you leave all other plugs in, and can regulate the line pressure, I've found you can do it. Don't really think you would have an issue with sudden air pressure loss. Much easier and quicker method.