Cat 3208 435 hp with Air Seps
Thanks for the question. First you need to verify if you are measuring crankcase pressure in the correct units. You should be using a manometer showing "inches of water"....if PSI, that is not a good diagnostic scale, it may signal a bigger problem.
Once that is resolved, I would take readings at full or cruise RPM, and record reading with AIRSEP...then remove the breather hose temporarily from AIRSEP, the record crankcase pressures. This way you can see what teh difference is with AIRSEP connected. Next, for diagnostics...since you have an engine exhibiting no issues....swap out the the AIRSEP units. Port to Starboard..and vice versa. If the problem does not transfer to the "good" engine...then you need to look at other mechanical Port engine issues.
If the condition does transfer to the Starboard engine...then you need to look at the AIRSEP...how old is it, what is serial number...etc.. the material inside may be compromised.
In general the AIRSEP will reduce crankcase pressure...depending on engine model and condition of engine will determine near zero or negative. It is also important that your engine has a specific 3208 AIRSEP kit, and that it has the proper size air filter for your application. It may need further fine tuning, but keep in mind it has limitations and will not mask or correct a mechanical issue with the engine.