Right, i'll try, but first, have you tried cross checking with their CD?
The metering unit meters air going into the VSI chamber.
If you just had a gaping hole in the instrument, the pressure differential would not really occur with height change because the pressure would equalise to quickly.
The metering unit restricts air flow so the differential can measured and seen by the pilot.
i.e., climb, into lower pressures, so capsule inside has greater pressure inside as a pressure differential is created, this compresses the capsule, which moves the needle to show an ascent at an appropriate rate which is based on the rate of change of pressure, i.e. metering unit is only allowing a set equalisation of pressures, so the original pressure inside instrument is kept until a higher height (which is at a lower pressure, so great ROC is shown).
To be fair, you are unlikely to get many if any questions on this! Gyros and EFIS are the big cookie, and, if your having problems with Gyros, don't worry, Baz will sort you out with his Bongo in the Congo.
If ya get stuck on anything else and you found my help of any use, feel free to email me with any more queries and i'll see what I can do! Alternatively - use the school your paying! they dont miund if you email queries or drop them a line.