The vacuum line is 7/32" inside diameter. Might not be a bad idea to bring the switch with you to get the correct diameter line, make sure it fits tightly on the nipples. Most auto parts stores should stock it. When you remove the old lines, take note which position the lines are in, or do them one at a time. The 4WD will not work properly if the lines are not on the correct nipples. Any line that had oil in it needs to be replaced. You'll need to measure for length.

 

If the transmission fluid level is not going down, the transfer case input shaft seal is not the problem, and the transfer case does NOT need to be removed.

If there are no visible leaks from the transfer case, the vacuum lines have fluid in them, and the level keeps going down, there is only one explanation. The vacuum switch is bad, (or the original one). If the switch sucks fluid into the lines, you will have difficulty getting in/out of 4WD.

Yes it is possible to overfill the transfer case.

 

Ok thanks, that's good news. I will go that route first, replace the vacuum lines that I can, clean out what I can and then see if I get get back into 2 wheel drive.

Thanks! and I will update this as soon as we are finished.

 

Make sure you check for fluid in the vacuum line to the front axle actuator under the battery. If there is any sign of fluid in that line, replace it, and remove the actuator and clean it out, or replace it. The vacuum line runs from the vacuum switch to the top of the engine. Then along the right side of the intake, behind the alternator, and down to the actuator.

 

I had this happen once and i replaced the vacuum switch , all the lines, the vacuum reservoir flushed the lines to the hvac actuators and had to replace the selector switch that distributes the vacuum to the actuators because the seal puffed up and wouldnt work anymore, after fixing everything the new vacuum switch sucked fluid up again and ruined everything for the second time, i didnt feel like pulling the transfer case so i just got rid of the transfer case switch all together and locked my front diff actuator in permanantly by wrapping a couple zip ties around it(works fine this way, the AWD bravadas and astros run the exact same front axle locked in full time), i now have a more reliable 4 wheel drive system and no chance of sucking up trans fluid into the hvac system or engine. (Yeah so the front driveshaft spins all of the time.... but for the life of the vehicle the extra couple dollars of gas every 100 miles Far outweighs the hassle of pulling the transfer case to change a 4 dollar seal.) I pull my fill plug at every oil change and top the transmission off, usually not much comes out or needs to be added, seems like the vacuum on the switch was causing the fluid to be sucked into the transfer case, doesnt make sense because the t-case has a vent so it shouldnt be able to hold vacuum.
Anyway that was my solution to solve the problem once and for all, yeah it was kinda ghetto redneck-ish but sometimes you just need to simplify an over engineered system.

 

Switch has been replaced, we blew out all the fluid which there wasn't much fluid.