Problem # 1 - not a problem. That's just the sound of vacuum venting from the filtered end of the master valve.
Problem # 2 - now you need to start plugging lines and find the leak. The fuel door lock should be at the end of the line, unless (and I don't remember) if the door is in the fender near the C pillar then it might not be at the end. Either way, cap off the vacuum line to that lock, there should only be one since the lock is spring loaded and vented to atmosphere (so if the vacuum bleeds off, the gas door will be unlocked). Install a vacuum gauge somewhere where you can keep an eye on it, run the engine or use a hand pump to build some vacuum and wait. If it holds, you found the leak, if not, disconnect and plug the trunk lock and repeat.
Continue the process of plugging lines until you hold vacuum 15 inches HG for 15 minutes, when you achieve this, you have isolated the leaking unit (keep in mind that one or more of the units already disconnected may also be bad). Once you get to the point you are holding vacuum, keep the last unit you tested isolated and reconnect one of the previous units. Check and see if the system still holds vacuum, repeat until all vacuum units have been tested and verified.
You might want to disconnect and plug the line that goes into the left fender, that should be heading to the vacuum resevoir. Taking the resevoir out of the system for testing will make even minor leaks very visible as the vacuum stored within the lines is nil. ALong the way expect to replace a few connectors between the vacuum units and the hard plastic tubing. This is just rubber hose vacuum line cut to make the connectors. I used 5/32" line, slightly smaller ID than the metric but makes for a nice tight union.
It takes a while to go through all of the lines, connectors and diaphragms, but in the end it's worth it.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.