Air Leak in the Cab?

klippencott · Feb 12, 2006

As most of you know I bought a used FL70 and it is the first truck I have owned that had air brakes, so I don't really know what to expect.

When I turn the engine off I can hear what sounds like air bleeding down under the dash. It is not a loud noisy sound but rather faint. I know I have a couple of air controls in that area that could be leaking. My brakes and air bags work find and the air pressure is steady when it's running. Is this normal or should I have someone look at it?

highway star · Feb 12, 2006

Any air leak should be checked out. Minor leaks are'nt a big deal, but they sometimes become major leaks.

LDB · Feb 12, 2006

I wouldn't go out of service somewhere out of town where I'd have to get a motel room, unless it gets much worse, but next time you're home for a few days anyway I'd get it looked at. Good luck.

Leo Bricker, owner trucks 3034, 4958
OOIDA 677319
73's K5LDB
Highway Watch Participant, Truckerbuddy
EO Forum Moderator 1+ Years of Service
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ATeam · Feb 12, 2006

The leak may be behind the dash, or on the other side of the firewall (the wall between the dash and the engine compartment). The firewall has a number of pass-throughs (holes) through which wires, cables, hoses, and tubes go through. An old mechanic's trick is to use a length of garden hose to zero in on the leak sound. Put one end of the hose close to your ear. Probe with the other end to isolate the area where the sound is coming from.

Of course, a GOOD freightliner dealer should be familiar with the symptoms. The sound could be a normally-functioning valve.

SHARP327 · Feb 12, 2006

Try squirting a mixture of dishsoap and water with a spray bottle on some of the hoses in the area where the noise is coming from, I don't think I'd spray the mixture under the dash due to all the electrical equipment but in the engine compartment and under the truck should be ok!

Packmule · Feb 12, 2006

klippencott,
Check the air bag valve. It has O rings in it that wear out very quickly because they are very small. If you don't replace it will blow completely out and one day you will lower your air bags and they won't come back up or they may leak down in transit,,,Talk about a rough ride!!!

Dan

klippencott · Feb 13, 2006

Thanks to everyone.

Dan, I think you may be on to something concerning the air bag valve. The air noise seems to be in that area, but the air brake control is close by also. I'm gona try the hose to the ear method and try to locate it.

Got to make a run!

RichM · Feb 13, 2006

The air brake valves in the FL series have always caused minor leaks, try this,charge the tanks up to the shut off pressure,turn the engine off. Now listen for the hiss,push the valve in and see if the hiss goes away.If so pull the valve open again to set the brakes and see if the hiss comes back.
As the other folks have said,best to get it taken care of as little leaks become big leaks in the worst possible times and locations

The Gibster · Feb 14, 2006

We had the same problem a couple of times, and it ended up being the airbrake paddle valve above the airbag switch. Like PackMule said, the little o-ring compression fittings wear after a while. Contributing to that is their close proximity to the heater ducting.

Our plastic lines at the ends had dried out over time and the o-ring fittings had begun to leak. You can compress the female fitting on the valve itself and remove the tube and o-ring. We actually cut a little of the tube off with scissors and reinstalled the o-ring and tube into the valve.

Just cut a little of the tube, like about 1/8". We nibbled away on it twice until we got to good tubing. Reassembled it all and it's been leak free since last summer.

Hope this helps. It's a sinking feeling when you hear the leak for the first time, but it's a fairly easy repair.

The Gibster

klippencott · Feb 14, 2006

Excellent information! I'm going to work on it Thursday. If your right I will be able to fix it myself, thanks to the guidance I've been given.

Thanks,

mhoy40 · Feb 14, 2006

i have a fl80 that just had the same type of problem it was the air line that gos to the back of the air tank pressure gauge the seal on the comprossion fitting just wore out easy fix though about ten minutes and a dollar and 2 cents later its fixed just get a 1/4 inch line compression fitting from the dealer take the 4 or so screws that hold the insturment panel on take a 7/16 open end wrench take the old fitting of cut the air line at the point were it goes into the fitting put the new fitting on the line and hook it back up. apperntly the fl70s,80s and the 106s are famous for the origanal fittings gettig wore out just enuff to leak.

klippencott · Feb 26, 2006

Progress Report:

Thanks mhoy40 and everyone else that helped me save a bunch of money on this repair!

I pulled the dash instrument panel off today and found that I had two leaks. Both were at the back of the air gauges. Just as mhoy40 had said. I cut both off, went to Freightliner and got new fittings, cost was two dollars and some change. It took me a little longer to put it all back together cause I replaced all the dash lights while I had it apart.

Again I say thanks for the expert help!