Time to start tracing out the fording system and the air system diagrams.The SEE student handbook in the Misc Tech Manal section has some simplfied air system diagrams. The short version is that air come out of the primary air system near the air tank where its routed preferentially to the brake booster system. If the air pressure is over a minimum threshold, and air regulator tapped off the main line opens and routes air to be used for various systems like remote throttle, air shifter, 4WD shifter and Lockers. Tapped off the 4wd air supply downstream of the 4wd selector valve is the fording purge system via another regulator, it drops the air pressure down to 5 psi and that is routed to portal axles to pressurize them to prevent water from getting in. If these hoses are broken or disconnected than the system will leak air whenever the 4WD is engaged.
A major caveat from my friend the Unimog "pro" is that the fording purge system has a dark side on the SEE and other unimogs like my 1300, if the regulator fails, it most likely fails at full pressure. In this case full pressure air is going to go into the purge system and overpressurizes the portal axles. If this happens, at a minimum the cup of gear oil keeping the portals lubricated blows out of the axle through the rear hub seal and then the portal axle is running dry and will self destruct soon unless refilled. Most likely the rear portal seal will be blown and then it will not hold the gear oil and that means tearing the portal axle(s)_ apart to replace the seals. His advice is unless you plan to be running your SEE underwater, disconnect the fording purge system. I have not done so but if and when the cab gets lifted, it is on my list. I will probably just cut in valve on the line downstream of the regulator.
