As you have been told, this should not be difficult for someone that is experienced.
We would address your problem first by verifying governor pressure and then removing the valve body and applying the various components with air pressure. Internal leaks become apparant and proper application (experience matters) assures mechanical integrity.
If both worked properly, we would look to the valve body and probably replace it with a used MB unit before we wasted too much time. They are usually less than $400 list price.
The key in the last paragraph is that problems become greatest if the risk becomes too great. One could spend hours/days chasing a
VB problem. The key from the shop/techs point of view is at what point does he go for the
VB and what does he do with it if it isn't the problem. This becomes a real burden to someone who does one of these tarnnies every year or less.