Replacing Valve Seals without Removing Head?

[savatreatabvr]

I've heard the best way to replace the valve seals without removing the head is to use compressed air, that works great if the cylinder doesn't leak. But isn't the reason valve seals are replaced in the first place is because they're leaking? An old old timer told me the best way yet, he said remove the plug, bring the piston to TDC and stuff the cylinder with 3/8" rope and the valve will not drop a fraction of an inch! When done replacing the seal just pull the rope out and move onto the next cylinder! I tried it and it works great! Be sure and spend a few extra bucks on good quality spring compressor, I almost lost some teeth when my cheap spring compressor broke and the spring came shooting out like a bullet! Got my attention!