I just swapped the hub engaging solenoid and will be returning it; wanted to rule it out. So its not that. Koeo truck vac comes on for about 10 sec ever 45secs or so...cant find any problems in vac lines nor lines at knuckles....im currently looking behind dash for the arm that switches it to vent.

 

Couple updates. I plugged black from pump to res stops instantly, disconnect from hub solenoid stopped cycling; held vacuum. Disconnect hub solenoid and plugged output from res tank, it no longer cycles every 45 seconds. Holds vacuum. Still no vent air or any other than defrost

 

. If you have defrost only in one of these diesels you know its most definitely a vacuum issue. It could be caused by a component that goes bad like the shift on the fly hub solenoid or the pump itself which is not heard of much in my reading of over 50 or so treads....
This is the order i recommend doing things1. Decide if you have a Vacuum leak.
Key on heat/air switched to off( so you can hear the vac pump easier) the pump should run for a bit and stop. The run time might vary a bit but id say even a minute is a bit abnormal....but if it stops running for a prolonged period; it would indicate your system is holding that vacuum with minimal leaking which is good. If the system is not believed to have poor vacuum skip reading number 2
2. If the pump runs continually it could mean 2A) a disconnected line somewhere which is very common under glove compartment. Just look from ur pump to the resevoir and the black out from that may have a rubber splice joint and then go through the( just trace these out with your hand easy enough etc). Black vac lines are the feeds unless someone did some repairs using any old line; like i did. 2B) from ur pump to the reservoir will be the vac supply. In the same boot that it goes in another line will come out; red line commonly. This red line supplies vac to the hub shifting solenoid( i dont know the lingo), its very very very common that this will be your issue. The vacuum lines that go the wheel hubs get supplied from this unit. Red line comes in from res black line goes out and branches to both wheel hubs....the bottoms of these lines convert to rubber tubing and is prone to crack letting in air and killing the vaccuum. 2C) this unit itself ive read can take on water that freezes creating cracks in it and destroying the ability for vaccuum too i guess...
2!)) After checking the hub areas for vac leaks and having them and fixin them or not having any; go back to the resevoir and disconnect the feed line to the hub solenoid unit. Plug up the hole tightly where it was plugged in. !!!This step is what solidified my slight vaccuum leak!!!. But even after i confidently established i had good vacuum i still had ONLY DEFROST...
3) Once confident you have good vacuum follow the single black feed tube from resevoir to first splice. Seperate here; should be plenty of noise evident that you just lost vacuum. Reconnect and let your system charge back up. Once pump stops empty ur glove box, open the press in the sides so the door dangles toward floor...here will be at fire wall a grey tube entering and the black feed tube right beside. Follow black tube and ull come to a connector where it splices to another black and a white tube also splice within this couple to another white...... disconnect the BLACK FEED !!!!!!!! If you dont hear **** your hearing blows or like me YOU HAVE A BLOCKAGE of vacuum somewhere from that splice you just checked that made noise and this one.......
4) From the spice that you know had vacuum disconnect the end further down the line from resevoir, connect new line approx 2.5 feet will prob be good but i used a 3ft one; and run that thing through the firewall and attach to the coupler where it splices with the black that you checked and heard nothing.

 

Yeah 3ft wasnt necessary

Where i ran my line in there was a grommet i just put a little hole in w drill

Pointing to where original feed line splice was that made a ton of noise when seperated

My new splice that eliminates the section of line that had a blockage

The screw i used to plug the feed to the hub solenoid

 

Lastly ( but firstly )if u have a continually running pump. Disconnect the line from pump that supplies vac to resevoir and press your finger on the now exposed nipple on the pump. It should cease running immediately. If not there is a prob with pump.