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Air System Constant Release

While inspecting a 1993 U240, I noticed that with the engine running but parked and air brakes set, the system constantly releases a puff of air about every fifteen seconds. What is the likely cause of this?
The coach has been sitting parked for some time and there may be issues related to that.
Thanks to all responders.
1989 Foretravel U300 Grand Villa
Build #3410

Re: Air System Constant Release

Reply #1 –
Releases the air from where?  The air dryer or some other location.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Air System Constant Release

Reply #2 –
Quote from: Chuck & Jeannie –
Releases the air from where?  The air dryer or some other location.
As yet unknown. It's the normal "hiss" of the pressure relief valve but I have not located it yet because I was busy with a long list of pre-buy inspection items. When I get to that next week, I'll report back.
1989 Foretravel U300 Grand Villa
Build #3410

Re: Air System Constant Release

Reply #3 –
Check Valve in air dryer bad.
2014 ih45  (4th Foretravel owned)
 1997 36' U295 Sold in 2020, owned for 19 years
  U240 36' Sold to insurance company after melting in garage fire
    33' Foretravel on Dodge Chassis  Sold very long time ago

Re: Air System Constant Release

Reply #4 –
Or, could be multiple leaks in the air system causing the compressor to run more than necessary to keep the system pressure up.
Richard & Betty Bark & Keiko our Golden Doodle
2003 U320T 3820 PBDS
Build # 6215
MC # 16926
2016 Chevrolet Colorado 4X4 diesel

Re: Air System Constant Release

Reply #5 –
Quote from: rbark –
Or, could be multiple leaks in the air system causing the compressor to run more than necessary to keep the system pressure up.
That's what I'm thinking. Will know early in the week when I get to thoroughly inspect it.
1989 Foretravel U300 Grand Villa
Build #3410

Re: Air System Constant Release

Reply #6 –
I highly doubt it's related, but my air dryer was popping off every few seconds a few months back.

Frequent air dryer purge
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Air System Constant Release

Reply #7 –
Quote from: stevej –
I highly doubt it's related, but my air dryer was popping off every few seconds a few months back.

Frequent air dryer purge
Thank you for this info.

Actually it sounds similar—too frequent purging, but I'm not sure which air drier is installed and, using your illustrations, will check that tomorrow. I have this coach scheduled into Western CAT and will have them look at it too.
1989 Foretravel U300 Grand Villa
Build #3410

Re: Air System Constant Release

Reply #8 –
When we got our '92 it had an AD-4 dryer. If you find that this is what is on this coach you might give some thought to upgrading to the AD-9. These are more modern and parts are more readily available. Several other options but you may have a bigger piping job to make some fit.

Mike
Pamela & Mike 97 U 320

"It's not what happens to you, but how you react to it that matters."

Re: Air System Constant Release

Reply #9 –
Quote from: Pamela & Mike –
When we got our '92 it had an AD-4 dryer. If you find that this is what is on this coach you might give some thought to upgrading to the AD-9. These are more modern and parts are more readily available. Several other options but you may have a bigger piping job to make some fit.

Mike
Mike—great advice. Thanks. I suspect that the quick release intervals indicate either the purge valve or something amiss with the air tank. I'll know more after Caterpillars inspects on Wednesday, but if I do have to replace, I'll definitely plan to upgrade to the AD-9 per your advice. Is that a 1:1 swap on the plumbing?
1989 Foretravel U300 Grand Villa
Build #3410

Re: Air System Constant Release

Reply #10 –
Quote from: rbark –
Or, could be multiple leaks in the air system causing the compressor to run more than necessary to keep the system pressure up.
15 seconds is wayy too frequent for a compressor cycle .  The fastest the compressor can build from cut in pressure of 110 to 125 is maybe 1 min at 2000RPM  .
  Mine did about the same thing and the dryer had an internal leak  into the purge chamber .  Swapped dryer and good.