Differential pressure

[BenRoberts]

Just curious if it is normal to have a release of air pressure when removing the fill plug on the differentials on my 923. I have been reading of clogged vent lines making the axle seals leak(both left side rears on mine) and all 3 of mine did it when removed so I thought it may be just the way they are.

 

[73m819]

NO not normal, Plugged axle vents, the pressure WILL find the weakest seal, then leak, if the pressure is still there, the next weakest seal will leak, ect., ect.

 

[Floridianson]

Ron remember the vent is made to hold maybe 1 pound of pressure in the axle so yes they do that with the stock set up vent when working correct. Now if it was a lot of pressure than yes the vent is not working as it should.

 

[BenRoberts]

Is it typically something obvious like mud on the vent or a discrete issue as in a check ball in a vent cap?

 

[73m819]

The issue is that the vent is designed to expel air, in this case the exhausted air is vapor that is formed from hot oil, the vapor is a tiny bit oily, this oily vapor makes the valve seat and the spring sticky which dust sticks to, over time this dust builds up to a point that prevents the valve from opening (leaks), in the mean time, it takes more and more pressure to open the valve (start of leak)

Something else to think about, the vent valve is a ONE WAY valve, it exhausts excess pressure, BUT does not allow air in when the axle cools down causing a VACUUM, air has to be drawn from somewhere, the seals are the ONLY place to get air from, so now the seals are having dusty air being SUCKED to the sealing surface.

The vent valve is there for one reason and one reason ONLY, along with the inner seal, they are used so when the truck has to came ashore from a amphibious off loading, water does not get into the rears and gear boxes, this is the SAME reason for the grease hubs instead of wet hubs. (water does not mix with grease)

 

[Floridianson]

Is it typically something obvious like mud on the vent or a discrete issue as in a check ball in a vent cap?

It's just a little spring and a piece of rubber. If you don't over think it then that slight pressure the OP heard means the input and output seals are holding good. Also when the axle cools the truck would be shut off and not moving so no dust.
Now I could ask the OP when you checked the rear was the axle cold. I have checked them cold and they still had small pressure so no VACCUM for dust or water.
Leave the vent there if it is working correct and had just a small amount of pressure. If you want to run a rubber line higher still leave that vent on the high end. This is so if you go fording the axle will have pressure and less like to take on water. I bet $100 bucks that if the axle had the correct small positive pressure hot and you went fording for five minutes the axle still would not cool down enough to suck up water because of a vaccum.

 

[BenRoberts]

one way or another im going to have to change a few seals since now 2 drums are full of oil. Time to get my hulk on since I lack the items to do the lifting.

 

[Floridianson]

Most wheel seals go what I have found is the mechanic did not tighten the bearing tight enough and the outer seal will leak. Yes a plugged up vent can put pressure on the outer seal. Most trucks of our size call for 100/150 Flbs to set the bearing and then back off. Then even the TM calls for going to 50 flbs then backing off 1/4 turn and lock it down. I think this is too much and the back off should be 1/8 turn. That will keep the outer seal tighter.

 

[BenRoberts]

So which seal do you think is letting the fluid in the drum? I just ordered the one on the back side of the drum figuring it was typical of how a normal 3/4 or 1 ton pickup was set up.

 

[Floridianson]

The seal that is to the outside or the one you put on last before you put on the bearing nut is the one letting the oil in the inside is letting it out so figure doing both.

 

[BenRoberts]

i referenced the chart in the stickey but did not see a number for the outer seal. do we have a napa # for this by chance?

 

[BenRoberts]

I got a newb question. Where is the vent at? im used to vents on pick-ups and what not and did not see it. im sure if it were a snake it would of bit me.

 

[Floridianson]

It is small little thing screw into the top of the axle. If you want to test it just remove it and if you can with your mouth blow through but not suck it is working correct. If you never plan on fording or taking the axles under water then you could extend with hose and small filter on the end. No no NAPA # I know of.
Yea the snake bit you.

 

[BenRoberts]

It is small little thing screw into the top of the axle. If you want to test it just remove it and if you can with your mouth blow through but not suck it is working correct. If you never plan on fording or taking the axles under water then you could extend with hose and small filter on the end. No no NAPA # I know of.
Yea the snake bit you.

yea i felt like an idiot down there

 

[BenRoberts]

another question, i ordered outer seals for the rear but am thinking if leaving them out as wet hubs. seems like a cooler running option. any thoughts?

 

[Floridianson]

We have discussed this and it has been done. The trouble I have is the axle fill plug is not set as high as the true floating hubs on the civi trucks. I like grease but too each his own but as said grease does better in water.