Tested egr valve with hand vac one day and it held a vacuum but really couldn't see if shaft was moving up and down. Couple of days later I had the carb. off and thought now would be good time to check the egr. Put the hand vac on and pumped and pumped and could not get it to hold a vacuum. I thought huh, I guess it just went bad in last two days. Took egr off and hooked hand vac back up, it works fine! Decided to clean it up. When I pull a vacuum to open up the valve it holds fine until I blow some compressed air into the port and then it all of a sudden releases and shuts tight. I don't have to have the air nozzle pushed tight into egr opening, about a inch back from it. I can't figure this one out. Is this normal, and why.