I have a 1994 Chevrolet G20 5.7 v8 3/4 Ton Van. It started to run rough so I took out my older egr valve and tested it the way you show and it didn't hold pressure and so I went to the parts store and brought my tester with me and tested 3 egr valves they had and they ALL tested bad. none held pressure. The shop owner thought I was doing it wrong he even checked my vacuum tester to make sure it was working and it was. I even showed him the video, so my question is am I doing it wrong? No one believes me that all three new ones could be bad. This is what I did.  I took it out of the van and put a vacuum pump on the top tip and pumped the tester up and held the pressure and the needle slowly fell. It did this on mine and all 3 from the parts store (these were worse than mine). What am I doing wrong?