I have been using a Supco Micron gauge and I noticed something odd from day one.

After my vac has run for a bit, I shut off my manifold to see if the numbers go up. If I were to shut off my manifold say around 1500 microns the numbers will sometimes go down to say 750. My Micron gauge has an aux port and a vac port. I hook the hose from the low side service port into the aux side, low side hose coming out of vac side to manifold. I have both valves open when pulling a vacuum.

What is causing my numbers to drop when I close my manifold? I'm guessing that my high side has a deeper vacuum and it is siphoning the from low side causing the numbers to fall.