Duke is right. No way air is going to stay perfectly sealed. One of the compressors jobs is to keep it at a given level throughout the task.

I may stand corrected, but I could swear I recall the pressure spec being 5 bar(approx. 75 lbs).

You may have seen this, but here's a thread that may help. There are many. Search for the word "stem" and "titles only".

http://www.peachparts.com/shopforum/showthread.php?t=78265&highlight=stem+bar