HI,
If you are going to replace the regulator O rings may I suggest that you watch the video below. Sub12Airgunners are among the best at explaining how to do things on airguns. I also suggest investing in some "Jeweler's Glasses" to help you when working on the O ring in the receiver body.
By the way, based on your description of the problem it doesn't necessarily have to be the O rings. Watch the video and he tells you where the air will leak for each O ring failure. Your problem might be the regulator piston. If you have an older rifle it will be made of delrin. The newer ones have a brass piston with a delrin cap. If the mating surfaces are worn or gouged, then the regulator will not function. That sounds like your problem. By the way a whole new regulator isn't all that expensive (relative to how expensive the rifles are).
Cheers,
Greg
Here is the regulator video. It shows work on an Impact, but the regulators are the same as in the Crown MkII and late MK 1's. If you have an early Mk I Crown you will have a smaller diameter regulator. Utah Airguns can tell by your serial number what parts you need.
This is the Jeweler's Glasses I bought. I found them very useful.