I just got my truck out of the shop aNd one of the things i had them check was for an air leak. I lose most of my air on the secondary gauge when i park and shutdown the engine and lose even more when they run a forklift on to load me...i have to let it build back up before i can move. Freightliner is trying to tell me i have a leak at the fan hub and its an 800.00 repair job?? Any input would be appreciated. Frank
Air leak
- Thread starter FJK1954
- Start date
greg334
Veteran Expediter
I am going to assume a couple things, one is you have air suspension in the rear and you may have an M2 or FL60/70/80/??
AND seeing you mentioned that this happens when you are being loaded/unloaded with a lot of weight, I assume you are not using your dollies or dropping your suspension.
If so, here is where it could be;
The air level valve under the box
one of your air bags is really leaking
the parking brake valve
I would say it is the first one.
BUT with that said, it may also be a bad manifold (a place where all the hoses hook up. I had to tighten the hoses on mine up.
AND seeing you mentioned that this happens when you are being loaded/unloaded with a lot of weight, I assume you are not using your dollies or dropping your suspension.
If so, here is where it could be;
The air level valve under the box
one of your air bags is really leaking
the parking brake valve
I would say it is the first one.
BUT with that said, it may also be a bad manifold (a place where all the hoses hook up. I had to tighten the hoses on mine up.
If it is a M2 it very well could be the AMU, air manifold unit. Read back in some old posts in here and you will see that it is VERY common problem. Freightliner is aware that the part is no good and has done nothing to fix the problem. I have replaced 2 in two years and this one should be going soon. It was last replaced in October. 
I don't have any idea. I am NOT very well versed on trucks in general, only know about what has happened on mine. I unfortunatlly learn by experience.
greg334
Veteran Expediter
Yes. It is a part that has a small gasket or o-ring that wears out.
P51bombay
Expert Expediter
The M2 I drove did that at time too - had it checked but they didn't find anything. Later on I got under there and wiggled the lever on the height control valve and sure enough - you could hear air leaking when the lever moved fore and aft. It never got worse or bad enough for the owner to care so its probably still like that.
termite289
Expert Expediter
posted and deleted cause i couldnt figure out where i saw fan hub.
then i found it again in the original post. my fan hub went to leaking this past winter. the air compressor had a hard time keeping up with the leak. the secondary pressure gauge would run up to 120 and back down to 90 in about 30 seconds. the compressor was constantly coming on, kicking off, and coming back on. the problem stopped when i flipped the switch to turn the fan on all the time. because in my freightliner, the fan needs air to be in the off position. if you turn it on, you stop applying air to it. thus you can test freightliners $800 theory that way.
if it is the fan, which i think you would notice the gague fluctuating wildly while driving. get a kit and rebuild the fan hub. for the press that they insist you absolutely must have to get it back together, you can use a couple of wood shop C-clamps.
i rebuilt mine in about an hour and a half, start to finish on the tailgate of my pickup truck.
then i found it again in the original post. my fan hub went to leaking this past winter. the air compressor had a hard time keeping up with the leak. the secondary pressure gauge would run up to 120 and back down to 90 in about 30 seconds. the compressor was constantly coming on, kicking off, and coming back on. the problem stopped when i flipped the switch to turn the fan on all the time. because in my freightliner, the fan needs air to be in the off position. if you turn it on, you stop applying air to it. thus you can test freightliners $800 theory that way.
if it is the fan, which i think you would notice the gague fluctuating wildly while driving. get a kit and rebuild the fan hub. for the press that they insist you absolutely must have to get it back together, you can use a couple of wood shop C-clamps.
i rebuilt mine in about an hour and a half, start to finish on the tailgate of my pickup truck.
greg334
Veteran Expediter
It's not the fan, he said it did a leak down when the engine was off and when there is a load on and off the truck (forklift), which means the air leveler valve, the air bags or the parking valve first in that order. I would replace the leveler valve first out of just a process of elimination, I have had the same problems and it happens a lot.
could also be the brake cannister, my fl-80 had a leak that would get a little wosre when i was getting loaded or unloaded with the truck off and dollies down i would have to start the truck let it build pressure back up to raise the landing gear,wouldnt take long usally by the time i made it from the driver seat to the landing gear it would build up enough to lift the gear,drove me nuts till it got a little worse finally pulled into a ta when i could hear what i thought was a air leake in the brake knob on the dash,mech. at the ta said it was a cannister changed it out air leak gone.was like a 40 dollar fix.
Slo-Ride
Veteran Expediter
Follow Gregs and Layouts lead first,,since ya are stating its when a driver enters your truck and ya dont dump the air,,,the weight of these forklifts can damage internal parts on the air system by forceing air back in the wrong direction..My M-2 leaks on the secondary side also after filling the air bags as I pull off the docks,,,seems if I cycle the manual switch to dump the air a few times it seems to build and maintain proper air...I to had freightliner look into this but got no help.
As far as the leveling valve,,if ya do force it move you propely will be hearing air thats what the valve does..Maybe a release valve sticking shut???not letting the air out fast enought??
The part bout leaking when shut down and parked could be other issues beside the air ride,,If ya got shop air at the house fill tanks that way and listen for other leaks..
Well good luck without being there and climbing under the truck,,,we are all guessing for ya..
G/luck,,,hope ya dont have to pay the shop again..
As far as the leveling valve,,if ya do force it move you propely will be hearing air thats what the valve does..Maybe a release valve sticking shut???not letting the air out fast enought??
The part bout leaking when shut down and parked could be other issues beside the air ride,,If ya got shop air at the house fill tanks that way and listen for other leaks..
Well good luck without being there and climbing under the truck,,,we are all guessing for ya..
G/luck,,,hope ya dont have to pay the shop again..
I use a 3/8 inch clear plastic hose as a stethascope to find my air leaks, amazing how well it works to find even the smallest of leaks. Layshooter, I know we talked about the M2's AMU's before. You may already know this but the AMU's are very easy to r & r yourself by removing the retainer bracket and sliding out the individual retainers for the defective AMU (the hose leak test works well to pinpoint which one is leaking). Oh, be sure and draw down the pressure first and have a tech sign-off on the repair when done. Save the labor cost anyway.
retidepxe
retidepxe