740 turbo estate 1987 with ACC slider version.

Another summer with poor/weak AC is nearly over but I had some time today to do some digging around for vacuum leaks - seemed like the logical explanation….

Background
AC seems cold - will always come out the defrost vents, although not a huge amount of air
You can select auto - after a very long time it might come out of the face vents, but not very strong and will disappear after a while and go back to the defrost setting.

What I have done so far
Have checked all the vacuum lines in the engine bay - all seem OK, the nipple on the vacuum reservoir looks split though,
Checked under the d/s kick panel for vacuum leaks coming though the fire wall, removed the vacuum line and there seems a reasonable amount of vacuum (not exactly sure how much vacuum I should have here though),
While the AC was on I noticed that the black box next the diaphragms was leaking a lot of air, only some air was coming from the feet vent (pic attached)
I have assumed that this section should actually be sealed so no air can escape but not sure?

Thanks in advance!