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Old 01-16-2023, 05:32 PM   #1
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Air Brake purge frequency

I have a 2014 Tiffin Phaeton with a Freightliner Chassis. At last falls service visit I had the air drier serviced and the tech located two air leaks. One at a clamp to rear brake and the other in the rear trailer hose? Both were fixed and I put the coach in storage. On my snowbird trip from Michigan to Florida I have now noticed that the air purge is happening much more frequently. About every 30 seconds I can hear the air blow off. Very noticeable at rest stop with engine left running. Not so noticeable while driving but that is just because of the road noise. This is why I think I did not notice immediately after service visit. I know the purge is normal and I have heard it many times before but just not this frequently. Am I just paranoid or is this the new norm with no air leaks in the brake system. Brakes air up very quickly at start up and gages are steady at about 120 psi. Any comments would be helpful.
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Old 01-16-2023, 05:38 PM   #2
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I agree, this sounds too frequent. If you air up, then shut off engine, can you hear any air hissing as you walk around ?
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Old 01-16-2023, 05:41 PM   #3
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Purge should only happen when governor says wet tank air pressure reaches CUT-OUT PSI.


That frequency is determined by air usage by air brakes and suspension.


Every 30 seconds is WAY too frequently and indicates a problem. Could be a leak, could be a failed purge valve, etc.


Get it diagnosed and fixed.
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Old 01-17-2023, 10:56 AM   #4
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Can’t say that I have heard a leak. If I had a large leak I would expect to see the dash air gages drip but they are very steady. Even pumping the brakes does not move them much.
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Old 01-17-2023, 11:27 AM   #5
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If no significant leak, the air dryer is the most likely suspect.
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Old 01-17-2023, 12:37 PM   #6
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Most times its the small line from the governor to the air dryer purge valve leaking, causes your problem.

The air dryer can't leak after a purge, it dumped the air in it and the check valve stops backflow.

You have a third tank called a wet tank. It doesn't have a gauge. That's the one that has the leak, so you won't see it on the gauges.

Get it fixed, running the way it is will overheat and wear out your compressor.
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Old 01-18-2023, 05:39 PM   #7
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Thanks for the info Twinboat! Studied up on the air drier and it’s connections. Got under the coach to find it and get some pictures. Coach was off and it was a little bit of a tight crawl. While under, I took a close look at the 1/4” governor hose. I gave it a tug and tried pushing it into its quick connect fitting. It went in about another 1/8”!! Started up the coach and no more frequent purge! I’m guessing this it it. The plastic tube is quite taught. I’m guessing after my latest service it probably came loose. Think I will head to Home Depot and get some tubing and splice fitting. Will also get a 1/2” female pipe coupling in case I ever need to bypass the air drier in the event of a purge valve failure. Do you know if there is pressure on the governor hose when coach is off and air bags dumped? Is there something else I need to drain before I pull the hose out of its fitting for a splice? I did not hear any leak before it pushed it in and there was still 75psi on both gages when I started up the coach. Thanks much for the tip!!
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Old 01-18-2023, 05:40 PM   #8
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Thanks for the info Twinboat! Studied up on the air drier and its connections. Got under the coach to find it and get some pictures. Coach was off and it was a little bit of a tight crawl. While under, I took a close look at the 1/4 governor hose. I gave it a tug and tried pushing it into its quick connect fitting. It went in about another 1/8!! Started up the coach and no more frequent purge! Im guessing this it it. The plastic tube is quite taught. Im guessing after my latest service it probably came loose. Think I will head to Home Depot and get some tubing and splice fitting. Will also get a 1/2 female pipe coupling in case I ever need to bypass the air drier in the event of a purge valve failure. Do you know if there is pressure on the governor hose when coach is off and air bags dumped? Is there something else I need to drain before I pull the hose out of its fitting for a splice? I did not hear any leak before it pushed it in and there was still 75psi on both gages when I started up the coach. Thanks much for the tip!!
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Old 01-18-2023, 07:17 PM   #9
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The line in question only has air in it when the ungaged wet tank is above cut in pressure, about 90 psi.

The governor sends air, from the wet tank, to the compressor unloader valve and the air dryer purge valve when the pressure meets the setting about 125 psi. That air is held in the lines until tank pressure drops to the low setting about 90 psi. The air exhausts and the valves close to start building air again.

As long as the tanks are below 80 psi, no air will be in the unloader line.

If in doubt, just pump the brakes until the gauges are down low.
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Old 01-19-2023, 04:28 AM   #10
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I have a Freightliner chassis on my 2012 and it is only a two (air) tank configuration.

They are paired together so I'm assuming one acts as the wet tank as part of its duty and appropriately equipped to do so.

Does your Tiffin have a 3 tank configuration....to include a separate wet tank?

I've recently discovered a hissing noise coming from the tank area with the coach fully aired up and engine off, but have yet to find where the leak is coming from.

Hopefully, I'll get to it this weekend.
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Old 01-19-2023, 06:27 AM   #11
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Most use a 2 chamber tank for the wet and secondary tank.
It will look like it has only 2 tanks but there are 3 in the system.
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Old 01-19-2023, 07:55 AM   #12
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I wondered why the one tank had more fittings and seemingly air hoses than the other....thanks Twinboat.
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Old 01-20-2023, 05:03 AM   #13
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Thanks again. You have been very helpful!
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Old 01-20-2023, 06:54 AM   #14
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I had this issue. It was the governor.
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