Regarding the gauge jumping to 23Hg when you turn the key off and are conencted to the line...you might try "t-ing" into the line so that the shutoff valve is connected as well.

Perhaps there isn't enough volume getting pulled to shut the valve off. The volume of the mityvac is very small and vac can be indicated high whereas when you are pulling air from a larger area (the shutoff valve) the high reading would be lower.

Just a thought...