Removing the oil cap on a 77 won't make a difference in idle- if you look at the hoses going to the oil fill, you'll see that one hose is going right to the unmetered air filter side of the air box. It's only later model cars were the idle changes.
Yes, air leaks can and are a huge determination on how well the system works. Air leaks will cause a lean condition. It would be prudent to find and fix all leaks before you worry about pressures.
Do a few searches and you'll find procedures for hunting down leaks. Here's a great thread- second post has some links for trouble shooting.
http://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html