[Musicman]

My truck used to be absolutely “air tight.” I could park it and let it sit for two weeks, and when I’d start it up, I’d still have close to 120 psi in the system. Recently it started losing air at a pretty fast pace. Shut it off for more than a couple of hours and you have alarms going off when you start it up. I have noticed that when it is first started, the air dryer dumps air at an alarming rate for 15 seconds to up to a couple of minutes, which obviously is not normal. Also, it now purges air in an unusual fashion. Every few minutes while driving, it will steadily dump enough air to drop both air gauges 15 psi, then stop purging and the compressor will build pressure back to about 125 psi at which time it will do a normal purge with the customary short strong blast of air. I tried replacing the desiccant, but it has made no improvement. I have to admit to knowing close to nothing about air dryers, but it seems like a problem that could be caused by a sticking valve or worn seal, no? Any ideas?

[allan5oh]

You need to change the purge valve, also there may be valves in the air compressor screwing up as well (less likely).

[Kranky]

...And if you're losing air from the entire system because of a leaking purge valve while the truck is shut off, then you've got issues with the check valves between the "wet" tank and the other tanks upstream.

Once the "dry" tanks are pressurized, the check valves are supposed to prevent air from leaking back toward the "wet" tank and air dryer.

Or, the leakage while the truck is shut off could be a completely different problem unrelated to the purge valve/air dryer.

It may require some detective work to find the leak, but if there's another leak somwhere that's losing the air pressure within several hours, you should be able to hear it if the truck is in a quiet location and you go under the hood and under the truck and listen carefully near all the air lines and air valves.

(Be safe - chock wheels before going under truck.)

Fill up a windex type spray bottle with a mixture of Mr. Clean and water, and spray all the air lines, connections and air valve exhaust ports. When you see bubbles, you've found the leak.

[bob h]

Originally Posted by Musicman

My truck used to be absolutely “air tight.” I could park it and let it sit for two weeks, and when I’d start it up, I’d still have close to 120 psi in the system. Recently it started losing air at a pretty fast pace. Shut it off for more than a couple of hours and you have alarms going off when you start it up. I have noticed that when it is first started, the air dryer dumps air at an alarming rate for 15 seconds to up to a couple of minutes, which obviously is not normal. Also, it now purges air in an unusual fashion. Every few minutes while driving, it will steadily dump enough air to drop both air gauges 15 psi, then stop purging and the compressor will build pressure back to about 125 psi at which time it will do a normal purge with the customary short strong blast of air. I tried replacing the desiccant, but it has made no improvement. I have to admit to knowing close to nothing about air dryers, but it seems like a problem that could be caused by a sticking valve or worn seal, no? Any ideas?

"It is better to open your mouth and tell us what vehicle you're having problems with, than to be quiet and leave us in the dark".

...sounds like an econ valve issue.

[Musicman]

Originally Posted by bob h

"It is better to open your mouth and tell us what vehicle you're having problems with, than to be quiet and leave us in the dark".

...sounds like an econ valve issue.

Okay, '06 Pete 387, ISX 450.

According to Peterbilt, air dryer is model ADIS. Reman part # 5010696 and New part # 801266.