Help with a Air Leak?

[Papa Rick]

I have a 1985 Freightliner Day Cab 350 BCC, spring ride on both the tractor and trailer.

I lose all of my secondary air pressure within 4 hours of parking (pulling both valves at the same time. My primary air will go down to 60lbs in 4 hours and stay there.

I have tried to find the air leaks, but have not been able to yet, any suggestions on this? (Remember it is spring ride suspension, not air)

[GMAN]

Have you tried to listen to see if you can hear any air leaking while the engine is off? Finding leaks can be frustrating. Do you drain your tanks on a regular basis? I would check around the air tank. I have had a little dirt get into the relief valve and allow it to leak or bleed the air off. Also check behind the cab near the air lines. I had a slow leak there once. It nearly drove me crazy until I found it. I have also had a seat leak air intermittently. Try raising your hood and listen around the compressor and fan clutch. It could be something as simple as a loose fitting. One other thing you may want to check is the air lines themselves. I have had one leak and could not find it until I jacked the trailer around to get into a fuel island in California. It would only leak when it got in a bind. If none of the other things work, you may want to check behind the air valves. The lines coming up to them can occasionally leak where they connect to the valves. It may be difficult to hear them leaking. Lines can also become brittle with age. Since it is difficult to find, it is likely something very simple. Good luck. Let us know when you find the leak.

[allan5oh]

With what you described, I'm leaning towards something that's an accessory, does it have air windows/locks? Or check the seats?

[Kranky]

Fill a spray bottle with Mr. Clean.

Spray all air connections and look for soap bubbles forming which would indicate a leak.

If you still don't find any leaks, it could be that an air tank on the secondary side has a rust hole developing behind one of the attaching straps.

I've had this happen, and it drove me crazy trying to find the leak, due to the fact that the sound of the leaking air was dissipated due to being behind the strap and rubber cushion around the tank.

Only a few weeks later when the leak got much worse was I able to definitely find it.

A truck which is 22 years old, such as yours would be a prime candidate for this type of rust induced leak.

If it was the primary side I would say maybe it was leaking back thru a defective check valve thru the compressor, but if the primary side is staying up, then that would be ruled out.

[Papa Rick]

No air accessories to mention, all manual windows, door locks. But the seats are air, will check them real close. This only started about 1 month ago. I thought maybe something happened after being in the shop, but it did it prior to me taking it to the shop.

Where I park the Truck is on top of a mountain where the wind blows quite frequently, so getting to do this with little noise is hard.

With the secondary air going down so quick, I didn't know if it would point to certain items to check 1st or not?

[yoopr]

Try just pulling your tractor brake valve. See if that helps any and if it does you've narrowed down your problem areas.

[allan5oh]

Yeah I think you need someone that really understands how the systems work.

The way I look at it is this, the 60 psi in primary and 0 in secondary is a big clue.

There will *probably* be a valve between the two tanks, but if the secondary tank drops below 60 psi, this valve closes the connection between the two.

So basically what I'm saying is the leak is coming off that secondary tank. I don't know exactly what runs off it, your seat will definitely be off of it. Get under the truck and look!

It would help if you knew someone that fully understood that exact system on your truck.

[allan5oh]

Wait, freightliner. When you change your system from defrost, etcc.. do you hear sounds like flaps moving around? I'm pretty sure your HVAC system is air.

I know with my dads, if he leaves it on defrost, it will run down to 60 overnight. Try it in different positions!

I know he gave up fixing it a long time ago.

[Papa Rick]

When I change from defrost no air hissing sound to vent I can hear a small hissing sound. So when I shut it down at night I always leave it on the defrost.

[yoopr]

Try building up your air pressure to the max-keep your a/c off along with everything else-Turn off your engine and you should be able to find out where your leak is.

[SoCal79]

I know what you are going through, my 79 Pete leaks down at about the same rate as yours. One thing that I did do was to put an extra check valve at the compressor that helped alot. I am in the process of re plumbing all of my air lines they are old and when I sprayed them down with soap some of them slowly formed bubbles like they were seeping air and I had spent hours just soaping the connecters. It can be a long frustrating process,small victories and soon you will be there.

[yoopr]

wondering if maybe the compressor has a leak

[Jumbo]

Just a thought. My bosses truck did the same thing only in about 30 minutes. Turned out engine fan hub was the problem.

[pepe4158]

OMG Rick...my Freightliners doing the same thing, can hear a hiss under the dash unless I move to re-cycled air, when moved to re-cycled air no hiss....hmmm someone said its out of the heater? Plz e-mail me what your was id you find it!....oopss yours was gone with defrost....not recycled ;-p

[Papa Rick]

Finally Found both of my air leaks.

1. The air governor was starting to go bad and leaking down, as well one connection was not tight. This stopped the leak down very much.

2. The other one, I am not sure what it is called, but when I switch from defrost to vent, I hear the door flap shutting, but I hear a real bad hissing sound in the dash. I can switch it back over to defrost and it stops, any suggestions on this?

Remember this is a 1985 FTL, could there be some type of push rod valve that may not be closing?

[tweety bird]

Finding leaks can be frustrating.

You can say that again!