I went thru some thing much like this on our 78 CD... Drove me crazy.. .. The problem when I found it was the vacuum tubes where they go from the body to the doors.. where it bends all the time.. Inside those rubber covers that have the tubes and wires in them... The tubes had bent time after time.. and cracked.. Problem was some times it leaked and some times it did not.. Under a test with a test pump it would suck it self shut but open the locks and open the door to check some thing and it would leak down... I cut the tubes where they bend and put hose over the bend area where the cracks were... Darn thing almost drove me carzy... Almost replaced the whole AC vacuum system before I found the problem..

Ken