Why does my vacuum pressure jump up when I turn off my vacuum pump?
The moisture inside the system is still boiling off and is no longer being taken out or compensated for by the vacuum pump.
There could be oil that is holding moisture inside either the hose leading to the manifold or inside the block of the manifold.
Why does my vacuum pressure jump up when I close my high side & low side?
You have isolated the micron gauge from the system, you are now only reading what is inside your manifold and the hose connected to vacuum pump.