In addition to euro287's excellent info, I'll add that there are diaphragms in the door, trunk, and gas flap that will probably be the problem (of course, it can be leak in the vacuum line or fitting but not common). The diaphragms can be bought separately to rebuild the bad components. I had to rebuild all of mine over a period of a few months as the other diaphragms began to leak. Pretty easy to rebuild, the pain is getting the door panels off without damaging anything. There is special tool that is flat on one end in a U shape that is inserted between the door panel and the metal portion of the door and used to pry the panel retaining pins out of the door. There are several of these pins (as I recall 8 or so) and you use the tool at each pin. Unscrew the push button for the door lock. Also, the door pockets connected to the door panles (if you have these) have a plastic hook into the metal portion of the door, the hook is located on the upper front of the pocket. Once the door panel is loose carefully pull the panel and pocket backward to release the hook. It is easy to break the hook if you are not careful. Once the panels are off you'll find the lock diaphragms at the bottom of the door, just follow the rod connected to the push button lock down to the valve.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.