Rear Axle Differential & Geared Hub Vent Line Question

[McP47]

New member about a year in on owning a 1987 M1038 (Maine Rebuild 2007). Happy to have found this forum as it has helped to answer a number of my questions on common problems and issues that I would have had no clue about otherwise, so many thanks to all posters here.

Have been working through various PCMS over time and recently was having the rear diff worked on to resolve a leak and came upon a severed vent line (circled in the attached pic). My mechanic pushed air in each direction and it seems to be blocked at the hubs. Given it has likely been this way since I purchased, did not want to seal the system up just yet and risk blowing out the axle seals that were just replaced to resolve the oil leak.

I am assuming these vent lines are a closed system to allow for water fording, etc and I am trying to decide on how soon I need to address as I do not ford the vehicle or get into any serious off roading at the moment. I am assuming one of the risks would be submerging in water, mud, etc and getting water in the diff/geared hubs....is my thinking correct? Any other risks I should be aware of or other thoughts with that context?

 

[NDT]

Blocked at the hubs is normal, you are pressurizing a closed gear case by doing so. I would splice the cut line and go on with life.

 

[Retiredwarhorses]

Don’t blow air into the component, start at the component and blow in the opposite direction, blowing 90psi will blow seals.

 

[McP47]

Blocked at the hubs is normal, you are pressurizing a closed gear case by doing so. I would splice the cut line and go on with life.

Thank you ...guess I am curious how a closed system vents between the hubs and diff.

 

[McP47]

Don’t blow air into the component, start at the component and blow in the opposite direction, blowing 90psi will blow seals.

Thank you. I haven’t seen any leaks yet so hopefully the air blown in by my mechanic did not break any seals. Given it seems there may not be a block per NDT‘s reply, any risks from your perspective in splicing and moving on, or leaving cut for now until I can take apart and blow air the opposite direction to ensure no other blockage? I am fairly capable, but not a mechanic so everything is a learning experience.

 

[NDT]

Thank you ...guess I am curious how a closed system vents between the hubs and diff.

The gear cases are connected together with the black poly hose, ending up at a vent filter behind your engine air cleaner housing. This works great until you get vampire syndrome and everything gets filled with transmission fluid.

See post 14, Action reminded me the vent line DOES NOT end up at the vent filter behind the air cleaner, it goes directly to the air cleaner.

 

[springer1981]

You can easily remove the oil fill in the hubs and then check the line. That won't cause any back pressure or damage seals.

 

[McP47]

The gear cases are connected together with the black poly hose, ending up at a vent filter behind your engine air cleaner housing. This works great until you get vampire syndrome and everything gets filled with transmission fluid.

Very helpful...thanks again. Read about the vampire in the past...couple things going for me in that regard - I have a 218 T case and also 3 speed, which from various threads, seems to indicate less of an issue vs 242 and 4 speed.

 

[McP47]

The gear cases are connected together with the black poly hose, ending up at a vent filter behind your engine air cleaner housing. This works great until you get vampire syndrome and everything gets filled with transmission fluid.

Was thinking through a bit more...if the lines terminate at the vent filter behind the air cleaner, shouldn’t the air move through at least somewhat as the air has to eventually escape anyway? Or is it just too far, small enough diameter of hose that you would not be able to tell?

 

[NDT]

Was thinking through a bit more...if the lines terminate at the vent filter behind the air cleaner, shouldn’t the air move through at least somewhat as the air has to eventually escape anyway? Or is it just too far, small enough diameter of hose that you would not be able to tell?

I don't understand your question. As the gear cases get hot with operation, the air inside expands and is expelled through the tube vent system. When the truck cools off, a ever so slight vacuum is created inside the gear cases which draws air back in.

 

[McP47]

I don't understand your question. As the gear cases get hot with operation, the air inside expands and is expelled through the tube vent system. When the truck cools off, a ever so slight vacuum is created inside the gear cases which draws air back in.

I appreciate you hanging with me. I guess was thinking that the air should free flow no matter what though the hub...but now I get my error, the hub is is sending the expanding hot air back through the line so when air was pushed into the hub it creates a seal and pressurizes to your original point...so I guess there is no immediate harm in driving it until I repair the line as long as I do not try to submerge in water, etc.

 

[papakb]

I thought about this quite a bit when I first got my truck and was worried about the vampire. What I decided to do was to remove the poly line from the air cleaner and put an aquarium airstone in the vent line where it connects to the filter. I put a small vinyl cap on the fitting on the air cleaner to seal it up. This allows the system to breath as things heat and cool, keeps water from sucking back into the system and removes the slight vacuum created in the lines by the engine running. Haven't had a problem with it in 20 years!

 

[McP47]

I thought about this quite a bit when I first got my truck and was worried about the vampire. What I decided to do was to remove the poly line from the air cleaner and put an aquarium airstone in the vent line where it connects to the filter. I put a small vinyl cap on the fitting on the air cleaner to seal it up. This allows the system to breath as things heat and cool, keeps water from sucking back into the system and removes the slight vacuum created in the lines by the engine running. Haven't had a problem with it in 20 years!

Thanks for the advice! Trying to do my homework as I am not sure how or if the line was purposely severed for some reason and want to make the best decision when I remediate with potential vampire risk in mind.

 

[Action]

The component's vent lines hook together and then run to a brass fitting on the engine side of the air filter can. If you blow air towards a component, you will get back pressure or blow a seal (as stated above). if you remove a vent from something, you should be able to blow through it. The lines are thin, so it wont be like a drink straw. Some of the items that tie together are diffs, steering reservoir, transfer case, trans, hubs, geared fan drive (if equipped), plastic master cylinder (if equipped)
There is a small filter looking thing between the radiator overflow tank and the windshield. That is for the fuel system only.