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

Could the intake valve be leaking?

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2012 E350
2006 Callaway SC560

jamesjetton:
I just did my 91 560 SEL about a month ago. Changed stem seals but did not have to use air pressure. All I did was make sure the cylinder I was working on is at TDC. I used the TDC whistler to determine TDC, this way I did not have to remove the fan. I rotated the crank from below.

How did you keep the valve from dropping?

when you are at TDC, the most that the valve would move downwards is about a 1/4 of a inch. I had plenty of room to compress the spring and be able to drop the keepers in place. Actually, turning a cylinder to TDC on one head would also set another cylinder to TDC in the other head, so you can actually work on 2 cylinders(one at a time) with one TDC turn. I used a plastic piece of stick to push( on the metal sleeve) the seals down in place. I realize that having compressed air to keep the valves up would make it a lot easier to put the keepers back in, but in my case, it worked just the same. I just made sure that the car was secured not to move. Battery was also disconnected.

If you're using compressed air, you should leave the breaker bar/socket on the bolt and bungee them up so it wont move.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6