OK, I bought one of those hose valves that screws into the spark plug port to hold the valve closed so I can change my valve springs.

The problem is that when I hook it up, there is obvious excess air (minor, when I remove the hose, it takes a second to bleed down) bleeding off from the combustion chamber. The engine has not been running in over a month and I believe the oil in the cylinder walls has drained off, leaving no ability for compression.

When I do have pressure applied (with a small leak, not big), and try to compress the spring, the valve goes with it, and huge amount of air comes in through the open valve.

What am I doing wrong? How do I hold the valve properly using this tool? Maybe just the retainers are sticking to the valve? I don't want to remove the heads if I don't have to, I am confident the valve seals are good and that is not the issue.