low air pressure due to regulator dumping air

engelstad · 06-04-2016, 02:42 PM

Hi. The air pressure is around 90psi and I can hear it dumping air pressure at this pretty low level. Is the air pressure regulator adjustable or is it a sign that it needs replacement or could some contamination in the air line be causing this? I am away from home now so am worried if pressure will continue to get lower... thanks!

cadillackid · 06-04-2016, 02:48 PM

when you say regulator.. which part are you referring to? the pressure "regulator" or Governor is located right at the comrpessor itself.. it should not be dumping air there.. it turns the compressor on and off..

if you are under the bus and its the large round "Bendix" canister thats your Air Dryer.. and it sounds like the purge valve may be stuck if its constantly hissing air from the center of the device.. ...

90 PSI is when the governor should send air back to the dryer after a purge cycle occurs.. if the purge valve sticks the air gets dumped there and wont build up to the tanks..

Just need to know which part is dumping air and we can try to figure it out..

-Christopher

cadillackid · 06-04-2016, 02:49 PM

also absolutelty DONT DRIVE it if your pressure is lowering... if you hit 60 PSI your spring brakes will begin to apply and will begin dragging trying to slow the bus down... if you get between 25 - 40 PSI the parking brake will pop out and imnstantly lock your rear wheels..
-Christopher

turf · 06-04-2016, 02:53 PM

most air governors are set at 120psi. they can be adjusted some, but not by accident.

Crawl underneath and get a spray bottle with soapy water and start looking for leaks.

an air governor is a cheap enough part to replace, thats a good place to start on your fix. especially if that is what is dumping your air.

find out where the air is coming from.

engelstad · 06-04-2016, 03:06 PM

Thanks for the info. It sounds just like before when I think the regulator or governor would periodically dump air when the pressure was above 120psi or so. It doesn't seem like anything is leaking, just that the regulator or governor is dumping air every few min at 90psi or so...

engelstad · 06-04-2016, 03:41 PM

OK I see the air dryer and the air is not coming from there. I see the air governor a few feet further back and no air from there either. I can hear and feel the air bleeding off every min or so from a brass fitting from the rear left air tank right behind the wheel. There is wire running to this also. Is this some sort of pressure switch? I am not sure if the issue is from the electric signal sent to it or something wrong with it mechanically? Seems like if I push in the inner thing of this brass fitting then the air pressure will build up to higher. Suggestions of the issue or fix? Thanks!

cadillackid · 06-04-2016, 04:15 PM

take a picture of it.. sounds like a high pressure pop-off or possibly was an air-accessory hooked up at one time..

if you shut the bus off does the brass fitting bleed air down to about 60 and stop?

-Christopher

cadillackid · 06-04-2016, 04:23 PM

Quote:

Originally Posted by engelstad

OK I see the air dryer and the air is not coming from there. I see the air governor a few feet further back and no air from there either. I can hear and feel the air bleeding off every min or so from a brass fitting from the rear left air tank right behind the wheel. There is wire running to this also. Is this some sort of pressure switch? I am not sure if the issue is from the electric signal sent to it or something wrong with it mechanically? Seems like if I push in the inner thing of this brass fitting then the air pressure will build up to higher. Suggestions of the issue or fix? Thanks!

the air governor is normall attached directly to the compressor itself.. I put my bus away or id show you a pic of mine.. here is a link to one online..

D2 Truck Air Governor Bendix Midland Freightliner - Newegg.com

there is also a protection valve between any main air and any accessories.. like air-ride suspension.., air seat, etc.. a protection valve makes sure the brake system is at 65 or above before it lets air go to the accessory...

protection valves look sort of like this.. and there may be more than one..

-Christopher

Brad_SwiftFur · 06-04-2016, 06:34 PM

Quote:

Originally Posted by engelstad

OK I see the air dryer and the air is not coming from there. I see the air governor a few feet further back and no air from there either. I can hear and feel the air bleeding off every min or so from a brass fitting from the rear left air tank right behind the wheel. There is wire running to this also. Is this some sort of pressure switch? I am not sure if the issue is from the electric signal sent to it or something wrong with it mechanically? Seems like if I push in the inner thing of this brass fitting then the air pressure will build up to higher. Suggestions of the issue or fix? Thanks!

That sounds like a purge valve, it's probably on what's commonly known as the "wet" tank, it's task is to periodically let a little air out, draining moisture with it. Over time, these can get gummed up and stick open. The wire is from a computer/timer telling it to open (probably every few minutes to half-hour or so.) Shouldn't be expensive to replace. You *could* replace it with a brass plug as a temporary fix, but remember to occasionally drain the air tank if you do so (at least once a day, usually at the end of driving.)

engelstad · 06-04-2016, 06:46 PM

Ok, I turned off bus and the air release to reduce pressure when bus is turned off, does not come from the brass fitting that seems to release air a few times per minute when driving.

Also I put my finger on it for a few min to see if air pressure would build up and gauge didn't show pressure increase. When I took my finger off of it though the pressure did decrease. I am in rural Nebraska but I found auto parts store and replaced the air dryer. No change noted yet but barely drove it since then. Old air dryer was very gunked up though.

If the brass thing is dumping air, I was wondering if I could remove the wire going to it, but it doesn't seem to be connected with quick disconnect... ??

Brad_SwiftFur · 06-04-2016, 06:53 PM

Quote:

Originally Posted by engelstad

Ok, I turned off bus and the air release to reduce pressure when bus is turned off, does not come from the brass fitting that seems to release air a few times per minute when driving.

Also I put my finger on it for a few min to see if air pressure would build up and gauge didn't show pressure increase. When I took my finger off of it though the pressure did decrease. I am in rural Nebraska but I found auto parts store and replaced the air dryer. No change noted yet but barely drove it since then. Old air dryer was very gunked up though.

If the brass thing is dumping air, I was wondering if I could remove the wire going to it, but it doesn't seem to be connected with quick disconnect... ??

Air dryers are a fairly common failure item - in the last 7 years alone I believe I've had at least 2 replaced. Purge valves are a lower failure rate item, I cannot remember the last one I had replaced. The wire should plug in somewhere - if not on the valve itself, then maybe a foot or so into the wiring somewhere. Unplugging this wire most likely will *NOT* stop an air leak. It would be due to the valve being worn and/or gummed up.

engelstad · 06-04-2016, 08:40 PM

Thanks. So the air dryer filter replacement did not seem to help increase air pressure.

I did experiment where I manually held down the middle part of the brass fitting, where holding it down does not allow the air to be released as it is now releasing every 10sec or so at about 90psi. The result after about 2min at 2000 rpm is the pressure increases to 130psi.

So this is good but I fear if I replace this brass purging valve with plug, then the pressure might just keep on increasing to too high...

A bit more to add... over the last week or so the max pressure the bus could maintain has slowly decreased from about 125psi to about 85psi now. Also a few months ago, the issue was the opposite in that the pressure was too high, 150psi or so. All of the sudden in that case around 800niles into a trip, the pressure went to 125psi and stayed there, until about 10 days ago etc...

Suggestions? Thx!

engelstad · 06-04-2016, 08:52 PM

Also FYI this brass valve that is bleeding off air every 15sec or so is near the top of an air tank, making me think it is not for auto water removal and more for maintaining pressure...

engelstad · 06-04-2016, 08:53 PM

I can email picture of this brass valve if it helps. I can't seem to attach to this post... my email is jonathan.d.engelstad at gmail.com

Brad_SwiftFur · 06-04-2016, 10:19 PM

......... With your air system building up to different pressures, especially over ~120 PSI, it makes me think the air pressure governor is giving problems. It should "cut in" around 100 or so, and "cut out" around 120. This should keep your system between 100-120 without cycling the compressor too much.

This valve on top of a tank won't do much (if anything) to purge moisture from the system, with that information it makes me think it's a pressure relief valve which should open around 140-150. It should never open, when it does it usually means the system is building too much air pressure (or the valve itself is failing; the springs in them can get weak).

cadillackid · 06-05-2016, 10:04 AM

defiitely sounds like a pop off valve of some sort..

the governor should be cutting off the compressor at 120.. a which time you should geta quick "WOOSH " from your air dryer.. a valve any place else that is releasing air isnt really the norm... you may have an auto purge valve which is more unusual..

to post pice here open a free photoucket account, post the pic to photobucket.. then in photobucket go to library, and then click the thumbnail pic...

then when viewing your large pic.. go to the right and scroll down a bit.. click the 'IMG' link and it will copy the link to your pic to the clipboard..

then open a post reply here and paste that link.. we all can then view the picture in this forum..

-Christopher

engelstad · 06-05-2016, 11:02 AM

Disconnected the wire going to the brass fitting that has been dumping air at about 85 psi; no change, continues to dump air at low pressure.

Manually pushed the nipple button in the middle of the brass fitting while wife revved engine; pressure builds to 135 psi and holds. Thinking about getting a plug at a hardware store to replace brass fitting to prevent low air pressure bleed-off (until I am home and can replace brass fitting). I'm assuming the spring in the brass fitting is failing. What do you think?

engelstad · 06-05-2016, 11:45 AM

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engelstad · 06-05-2016, 11:47 AM

The brass part with nipple in middle is what is bleeding off pressure around 85psi...

cadillackid · 06-05-2016, 12:18 PM

so the large air line comes in through the frame? where does the small black air line go to? an eaccessory or rear suspension?

it looks like a pressure pop-off valve.. and you say there are wires connected to the top of it?
-Christopher

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