Something funny happened today. I was double checking the vacuum lines before I returned the rented gauge and the change over valve had no vacuum coming out of it. I think I found my problem. The valve is failing intermittently. I measured the vacuum twice before and it had 20 in Hg of vacuum coming out of it. All the measurements were during AIR pump operation to make sure the valve is energized. So I will replace the valve and report back. Thanks for all your help.