Hello, welcome and thanks for asking your question. My name is*****'m a Certified Auto Technician with 28+ years experience. I will do my best to assist you. please keep in mind that I may need to ask questions of you before I can answer. When you reply please feel free to add as much detailed information as possible pertaining to your concern.
I'm a fully experienced Dodge Technician that has worked on the Cummins engines in Dodge Trucks since day one. I was around when the first Cummins equipped Dodge Trucks were delivered back in 1989....
I myself have NEVER ran across an issue where the vacuum pump can pressurize the crankcase. I have seen vacuum pump seals fail many times and cause a very large leak to be generated at the point where the vacuum pump meets the power steering pump but in each case, the crankcase was not pressurized. In these cases, rebuilding or replacing the vacuum pump solved this issue.
As for what you are describing, it sounds like a classic case of excessive crankcase pressure buildup due to excessive blowby. Just because there is excessive blowby doesn't mean that the engine won't have plenty of power. Many times I worked on vehicles with this condition, there was never any mention of the customer complaining about a loss of power. The only concern was about large oil leaks and oil consumption. Sometimes, there is a clue upon initial starting of the engine in the morning during a cold start. I have seen many times where upon initial starting, the engine runs rough for 30 seconds or more and much white smoke out the exhaust. This is a clue that there may not be enough compression for complete burning of the fuel. Once the engine begins to warm, these symptoms clear quickly.
I have an easy way for you to check for yourself if there is excessive blowby caused by a worn engine.
When you start the engine for the first time, pull the oil fill cap before starting the engine and leave it off. Start the engine and let it idle. Look for puffs of smoke billowing out the oil fill cap. If you see much smoke and puffing up through the oil fill cap, this is a sure sign that there is excessive blowby caused by one or more failed cylinders.
This could be caused by "Dusting of the engine" which can be caused by poor air filter replacement intervals, or the use of aftermarket filters of inferior quality or the use of many of the K&N style air filters that are not properly maintained. Could also be that some piston compression rings have broken and caused loss of compression this way.
Keep in mind that the5.9 diesel is a fairly powerful engine, many times with this type of failure, you may or may not notice a considerable amount of power loss when problems like this arise.
if it is possible that you can make a video recording of you performing a "Cold Start" while recording the engine idling with the oil fill cap removed, I may be able to confirm that there is in fact excessive blowby caused by one or more cylinders...
Keep me posted...