Ooops! I forgot you had an 85 model.
The blue thing is a vacuum amplifier. With this device the value at full swing on the proportioning valve will not go to zero at idle. I would think it should go lower than 9in.
The difference with the amplifier is that the amplifier takes the amount of boost into the equation. If you put a long line on your gauge and watch it while someone else drives, you will see that full throttle will drop the vacuum to modulator to a value (9in in your case) and as full boost is achieved the vacuum should deminish to zero.
Your line breaking might have been a blessing as the standard reason for the vacuum being too high is the supply oriface having been reamed too large. This happens because soot gets into the oriface and plugs it. Cleaning it too vigorously enlarges the hole and supplies too much vacuum.
Bosch Master, ASE Master, L1
33 years MB technician