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Old 06-09-2019, 06:09 AM   #1
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Losing too much air pressure when applying service brakes

Hi, we recently purchased a used 2003 Allegro Bus and have an air leak.

There's no discernable air leak when the system is pressured up and the engine is off unless I press the service brakes. With the engine off, if I press and hold the service brakes down, I will lose air in both tanks at a rate of about 1 PSI / second (I.e. if I run the engine to get the tanks to 125 PSI, turn the engine off and leave it on accessories, the system will hold the pressure with no noticeable loss after several minutes (probably hours) but if I press and hold the service brakes down I can see the tanks go down rapidly (not just 1 time when I first press the brakes) and within a minute, I'll get the low air alarm). It doesn't seem to matter if I press lightly or heavily (although maybe I need to test that more) and both tanks drain at the same rate.

I'm going to bring it in for servicing but I'm just curious to know what to expect (I'm a bit paranoid about being told I need unnecessary repairs).

FWIW, I suspect the motorhome has been sitting for a long time (probably at least 6 months, possibly 1 or 2 years).
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Old 06-09-2019, 06:26 AM   #2
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I don't believe you have a leak and it is normal. In fact that is how you test the low pressure alarm. My rig will do the same thing if I press the brakes with the engine off. I can keep pressing the pedal till no air is in the system

*Test Low Pressure Warning Signal
To perform this test, the vehicle must have enough air pressure so the low pressure warning signal is off. The engine may be on or off; however, the key must be in the “on” or “battery charge” position. Next, begin fanning off the air pressure by rapidly applying and releasing the foot brake. The low pressure warning signal (buzzer, light, or flag) will generally activate when the air pressure falls between 55–75 psi, but may activate at a higher pressure (80–85 psi) if specified by the manufacturer. The low air pressure warning signal must activate before the air supply pressure drops below 55 psi in the air tank (or tank with the lowest air pressure in dual air systems). The vehicle is not safe to operate if the low air warning signal does not activate before the air supply pressure drops below 55 psi. See Figure 5.5.
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Old 06-09-2019, 06:31 AM   #3
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I don't believe you have a leak and it is normal. In fact that is how you test the low pressure alarm. My rig will do the same thing if I press the brakes with the engine off. I can keep pressing the pedal till no air is in the system

*Test Low Pressure Warning Signal
To perform this test, the vehicle must have enough air pressure so the low pressure warning signal is off. The engine may be on or off; however, the key must be in the “on” or “battery charge” position. Next, begin fanning off the air pressure by rapidly applying and releasing the foot brake. The low pressure warning signal (buzzer, light, or flag) will generally activate when the air pressure falls between 55–75 psi, but may activate at a higher pressure (80–85 psi) if specified by the manufacturer. The low air pressure warning signal must activate before the air supply pressure drops below 55 psi in the air tank (or tank with the lowest air pressure in dual air systems). The vehicle is not safe to operate if the low air warning signal does not activate before the air supply pressure drops below 55 psi. See Figure 5.5.
Thanks for the info but definitely not normal and the vehicle does not pass a pre-trip air brake inspection (With the engine off and brakes engaged, you should not lose more than 10 psi (after the initial drop) in 2 minutes" (or something like that - I'm sure different pre-trip tests use slightly different numbers depending on straight truck or combo trailer, length of test, possibly state/provincial rules in that area, etc).

If you press and release the brakes, you will fan the brakes and drop the pressure but if you simply press the brakes and hold the pedal down, you should not lose pressure after the initial drop.

There are lots of resources but scroll down the "Leak Test" here for an example (about 2/3 down the page):
https://www.smartdrivetest.com/air-b...brake-pre-trip
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Old 06-09-2019, 06:33 AM   #4
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Losing too much air pressure when applying service brakes

Sorry , pressing the brake down engine off you must not loose air pressure beyond the initial drop . Air brakes 101
You have a leak that if you were a commercial truck , would put you immediately out of service !!! No movement of truck and would require a mobile tech or tow to service.
Yes you pump the brake repeatedly to test the alarm and spring brake set multiple pumps will deplete the air but just holding the brake should be NO leak down

Ranger smith you also have a serious problem that needs attention from what you describe
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Old 06-09-2019, 09:22 AM   #5
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Have someone listen by all the wheels while you hold the brake peddle down. You will find your leak, and it is most likely a pancake in your brake chamber, or a plastic line with a hole rubbed in it! If you had to ask this, you definitely need to take it to a heavy duty truck shop, and they will fix this for you quickly! Rail!
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Old 06-09-2019, 12:36 PM   #6
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I agree, you definitely have a problem. Guardrail53's comment is a good way to get started. You have got a leak in one of the brake cans, the lines from the can to the antiskid control valve, or in one of the antiskid/brake control valves themselves. Considering how fast you're losing air it should be fairly easy to pinpoint the problem location by ear. Just be sure to put a block under each side of a rear dual before you stick your head under there. An another way to check would be to put it up on jack stands (not your leveling jacks) and crawl under to get a close up check on components.
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Old 06-09-2019, 01:18 PM   #7
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Thanks for all the responses. I've tried hearing it and can't tell myself. I can kind of hear it at one spot but it doesn't seem constant enough to be the leak. I originally thought it might be at one of the wheels but both tanks drain at the same rate which I believe means that it's before the systems split.

http://suzanneblackie.com/Pictures/a...ke%20lines.jpg

(all the damp spots are from me spraying it trying to get the grime off and seeing if I see bubbling).

In any case, I'm taking it to a Freightliner shop tomorrow morning. Thanks again for the suggestions,
Mike
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Old 06-09-2019, 01:19 PM   #8
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All above is great advice on where to start. Just for information, when you do an air brake test you depress and hold the brake pedal for one minute with the engine off. If there is a drop of more than three PSI the brakes fail.
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Old 06-09-2019, 02:54 PM   #9
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"With the air pressure built up to governor cutoff (120 – 140 psi), shut off the engine, chock your wheels (if necessary), release the parking brake (all vehicles), and the tractor protection valve (combination vehicle); and fully apply the foot brake. Hold the foot brake for one minute. Check the air gauge to see if the air pressure drops more than three pounds in one minute (single vehicle) or four pounds in one minute (combination vehicle). If the air pressure falls more than three psi in one minute for single vehicles (more than four psi for combination vehicles), the air loss rate is too much."
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Old 06-09-2019, 03:35 PM   #10
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During your air leak test you should not loose any air.
1) one of your service brake rubber diagrams has a leak in the brake chamber or air line going to the service brakes is leaking.
2) if your brakes are out of adjustment this will also take a volume of air until the brake shoes make contact with the drum then they will hold pressure. Brake adjustment maybe needed.
You need to have someone listen for air leak while your pressing on the brake pedal with the park brake released. ( chock wheels and on flat ground).
You really need a heavy duty mechanic to check your brake system out professionally.
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Old 06-09-2019, 05:20 PM   #11
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Brakes out of adjustment will use more air on application but will not continue to drop in pressure while holding the pedal down.
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Old 06-09-2019, 06:06 PM   #12
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Originally Posted by michelb View Post
Thanks for all the responses. I've tried hearing it and can't tell myself. I can kind of hear it at one spot but it doesn't seem constant enough to be the leak. I originally thought it might be at one of the wheels but both tanks drain at the same rate which I believe means that it's before the systems split.

http://suzanneblackie.com/Pictures/a...ke%20lines.jpg

(all the damp spots are from me spraying it trying to get the grime off and seeing if I see bubbling).

In any case, I'm taking it to a Freightliner shop tomorrow morning. Thanks again for the suggestions,
Mike
How far away is that Freightliner shop? It might be too far for safety if you have a leak in the air brake system.

Like others have said, that leak would put you out of service if you were a commercial vehicle. There are mobile repair services that can fix that on site. Replacing air brake cans, lines, fittings, etc. is not difficult for the guys that know what they're doing and shouldn't take too long once they arrive.

A small leak can become a huge leak instantly. Since you don't know where the leak is yet, you can't really know what the safety of the situation is.
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Old 06-10-2019, 08:04 AM   #13
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How far away is that Freightliner shop? It might be too far for safety if you have a leak in the air brake system.

Like others have said, that leak would put you out of service if you were a commercial vehicle. There are mobile repair services that can fix that on site. Replacing air brake cans, lines, fittings, etc. is not difficult for the guys that know what they're doing and shouldn't take too long once they arrive.

A small leak can become a huge leak instantly. Since you don't know where the leak is yet, you can't really know what the safety of the situation is.
Thanks for mentioning it but we're only about 3.5 miles from the shop. And, although it's certainly an issue, it's still pretty much driveable as the compressor pretty much keeps up with the leak so it doesn't really drop below 90-100 psi in regular city driving. They only time it becomes a problem (as long as the leak doesn't get worse) is if I'm stopped at a red light for several minutes as it will eventually drop to below 70-80 and approach low air area but that's easily mitigated by putting it in neutral with parking brake at red lights (and that gets it back up to 125 psi).

It's a the shop now - hopefully for an easy fix!
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Old 06-15-2019, 06:06 PM   #14
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Just went through this issue with our Freightliner XC coach. Had a leak, could hear the leak at or near the front axle and the front air tanks. Same situation where the air compressor would air up where we could travel safely. The air would leak down to 100 psi and then kick up to 120 psi. No problems with braking.
Went to the Freightliner Ossis service center in Gaffney, SC. Great service.
The tech replaced the inversion valve which is central to the air brake system.
Now we are ok and planning to continue to New York state.
Things happen with our rvs. Part of the adventure.
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