Lost all my air pressure overnight?

You said it. I'm chasing one air leak for almost 4 months now. It takes 24 hours to bring the tank from 120psi down to 60psi.

To the OP, the best way to go about this is by process of elimination.
1st thing you do is un-hook your trailer from the truck and see if that drains the tank(s). If it does then you know the problem is with the tractor. If the tanks don't drain then the problem is with your trailer.

I had a problem with my 2004 Volvo. It drained the tank very slowly however I could not hear the leak. When the weather got colder here in Canada I heard a faint leak, however I could not pin point it.
Finaly about a month later I could hear it coming from underneath the cab.

What I can say is that as long as your compressor can maintain the air pressure I would not worry about it to much until you can actually hear the leak..

 

The air leaks that are the hardest to find are the unloaders, But they will make the compressor cycle, not drain the whole system. Get a creeper and a spray bottle and spray every conection that is how most leaks are found.
If you are in the cab and or it is windy the cab can drain the system.
Just a thought!

 

I have done this and more. Once I found that bad air leak on the tractor air, I then verified ALL of the fittings on the entire rig from the back to the front. NOTHING was leaking. I suspect a bad check valve on the wet tank (maybe?) at this point...since nothing is creating any bubbles anywhere.

FYI - when the rig is parked in my driveway (45degree slant to the street) I loose most of my secondary air, and part of my primary air. But when I park the rig at the yard (totally flat)...even for a week, the primary air is FULL to 95PSI but the secondary tanks are EMPTY.

I replaced the sleeper air bags when I got the rig, since they did leak...but....haven't figured out what else there is here...
I even dumped the suspension when I let the rig sit and I still loose air..I figured that eliminates the air bags in the back...

 

if its not bad enough to see whaere its coming from, pry not bad enough to worry about yet. anything i cant figure out i let it go till it gets worse, the problem will eventually be easy to find!

 

Sounds like a Leveler valve Hanadarko.
Could be on the sleeper or the tractor. When you park in your driveway, get a spray bottle with some soapy water in it and spray it around your leveler valves. And on your cab air bags if it has them. Sounds like it is suspension though.

 

What! There are trucks that hold air pressure overnight? I've never driven one.

 

I agree but...the sleeper bags are brand new and the leveler is only 1yr old. When I soak them down, I cant find any air bubbles...but at this point, my money is on the leveler as being one of the culprits...I agree.

Bags used to whistle...once I replaced them ($55 each) that stopped that.

 

Air the truck up, shut off engine. Listen for air leaks, repair as needed. If system only drops 10psi or so in hour I would forget about it. All trucks leak air. Simply too many connections and fittings that aren't really meant to be 100% NASA approved air tight. Orings get hard, hot/cold expansion and contraction are the two most common problems. All you can do is spray soap/water solution on everything you can get too and hope you get lucky!

 

Check your "safety valve". Air the system up, shut the truck off, and climb underneath it on the driver's side just behind the battery box. On the inside of the frame (behind the battery box, and right about parallel to where the shifter goes into the transmission) there will be a "block" with about eight different hoses going into it. Feel around on that thing. On of the holes will be plugged with what looks/feels like a golf tee. This is where my "progressive" leak was finally found. It's the valve (block), and not the tee/plug. Simple fix, but about sixty bucks for the block. Just a suggestion.

 

My valve block doesnt appear to leak at all...

However, something odd has started happening:
Until about 2 weeks ago, i would loose only the SECONDARY air over the course of a few days - but the PRIMARY air was always staying arounf 100psi -even after a week.

Now, I am seeing both systems drop EQUALLY. I parked the rig in the same spot Sunday (as I always do)..(100PSI when I shut down) and 24hrs later it was 60PSI .
It wasnt windy - cooler but not windy.

Not sure what to make of this?!