The "spring compressor" pushes on the disk at the top of the valve to compress the spring As your spring is broken it may be partially "compressed" already. Here is a you tube with one possible solution. Note in this you tube the head is off and a table top is used to stop the valve from moving. In your case you'll be using air pressure to stop the valve from moving. Congratulations on figuring out it was a broken valve spring. If it were me - I'd swap the spring and get it back on the road, but then again I'm given to drilling holes in my IMS and running the engine up to 7K RPM daily.
how to remove/install valves with household tools - YouTube
All the best!