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Old 11-28-2016, 09:04 PM   #1
Mini-Skoolie
 
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Air leak??

I've got a leak in my system somewhere, but can't seem to locate it, or perhaps I'm missing something and need to know where to start.

Our compressor charges fine and cuts out at 120-125 it then slowly leaks to about 100 before it charges again. It exhausts fine and all other gauges and systems seem to be reading just fine. We took it to a an International service stop and the foreman walked around and soap tested all the major connections. No leaks that he could tell. He told us we'd be fine, but that still doesn't solve our prob.

So, would you start with changing the cartridge in the air dryer or replace the whole air dryer or what? We're on the road and have another 1,500 miles. I'm a little concern about such a rigorous duty cycle on the compressor (every 5-7 mins). Should I be concerned about messing up the brand new compressor? Thanks ahead of time for your thoughts!
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Old 11-28-2016, 09:23 PM   #2
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As long as you're able to brake with no issues, and if everything else seems normal you should be ok to get to where you're going. That duty cycle is somewhat extreme but as long as you don't run it for months like that I wouldn't worry too much about it.

It is a possibility that something in or around your air dryer is leaking but the International dealer should have checked that all out when they tested the rest of the system.

It's possible that one of your air lines may have a small crack or pinhole in it somewhere. I would suggest tracing all the air lines and doing the soap test on them, and see what comes out of that. Maybe hit all the air dryer fittings and the joint where the filter housing attaches as well.
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Old 11-28-2016, 10:18 PM   #3
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build up full air pressure and then shut off engine...does the air pressure in your gauges drop?

You need to have some idea where your air leak is before you get to comfortable with it....for safety sake
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Old 11-28-2016, 10:37 PM   #4
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OK, so if I'm reading this right, you're losing 25 PSI every few minutes while traveling. A little high (and borderline inspection fail, depending on exact leakage rate), but not quite enough to consider breakdown or stranded ...

First things first. What's the leakage rate when parked, stopped, engine off? Does it deplete to 0, or to some other pressure before reaching an equilibrium?

How long does it take to charge to max? Do you hear any audible leaks (may have to shut off engine to hear)? Leakage rate remain low when engine is running (parked), or unacceptably high? Does leakage rate change when parking brakes are released (assuming air brakes)?

In my experience, a failed or leaking air dryer will not build up pressure over 80 - and that's running the engine at high RPM's. The leak will be very audible if this is the case. With a 5-7 minute "cut-out" time, this is unlikely to be the culprit.

If you are maintaining air pressure when parked, but losing it when parking brakes are released, you may have a failed air line, or failed brake chamber (more specifically, a failed diaphragm within the brake chamber). Either one will be audible with brakes released (Do this with a trusted helper behind the wheel, wheels chocked on level ground, and whatever other precautions to avoid being run over with brakes released). With a vehicle over 10 years old, especially with brakes applied/released many times per day during it's school career, this is a fairly common failure.

When shut down, if the air leaks down to the 60-80 range and stays there, it indicates to me it's leaking somewhere past the protection valve. Basically, this valve remains closed until the air pressure reaches this valve's threshold, at which point it opens and supplies air to most of the "non-essential" systems ... air-ride (if equipped), some engine accessories, possibly fan clutch (if air actuated), stop arm and sign(s) (if air operated and still equipped). The brakes are usually set up to get air regardless of pressure.

Your ears will be the most valuable thing in tracking down air leaks. It is common for air systems to leak a bit - 2-3 PSI per minute when driving is not uncommon and can be considered normal (and as I recall, is the threshold for inspection pass/fail). If the bus has been holding air before and this is a new behavior, then you are wise to at least seek the cause.

I'll need a few more answers before I can offer a better guess, but if it were me, I'd be checking the rear brake chambers for hissing sounds (with brakes released).
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Old 11-29-2016, 12:43 AM   #5
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Thanks!

So we stopped and soap tested every connection we could find. Engine off and engine on. Both were losing pressure, although it loses pressure much faster now under a load (while driving) than at idle or off. We couldn't hear or feel any hissing either near the compressor or near the brakes, dryer or tank. I just don't know what it could be!?

It doesn't leak past 80-100 psi, but it slowly falls to that after charging at idle. There doesn't seem to be any probs with it recharging slowly.

We ran out hands along all the lines we could reach or see.

Also, it seems to be worse after this last stretch! It's recharging every 2 mins. So that's 25 psi per two mins. It used to be every 5-7mins.

Any thoughts anyone?
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Old 11-29-2016, 01:09 AM   #6
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Let it build up to pressure, then cut the engine and watch the gauges. Do you see one air system losing pressure while the other does not? Or are they both dropping about the same? And is it dropping regardless of parking brake on or off?

You mentioned that it was recharging slowly..... has it always charged slowly or is it now slower than usual?

If it's dropping 25psi in a couple of minutes you should definitely be able to hear it escaping.
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Old 11-29-2016, 01:30 AM   #7
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Both gauges always fall and rise in unison. It does drop slowly regardless of the parking brake on/off. Though releasing it sure takes a chunk out of the pressure.

The air loss only happens dramatically (10psi per min or so) when we're driving, so we can't hear the leak. When we're parked it's very gradually and we can't hear feel or see any bubbles. While it's a gradual drop w the engine off, it still seems like we should be able to hear it! Nothing...

It's certainly gotten worse throughout the day. Not sure what that could tell us?

The compressor recharges from 100 to cut out at 120 in 27seconds at 55 mph.
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Old 11-29-2016, 01:32 AM   #8
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I think I meant it recharges slowly compared to when we're driving. It's a steady climb at idle too, but slower than 27 sec for sure.
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Old 11-29-2016, 02:06 AM   #9
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That 27 second recharge rate at speed sounds pretty typical, some are a little faster or a little slower. And at idle of course it will take longer since you're only turning about 500-700 rpm as opposed to around 2k at speed.

When you release the spring brakes you're gonna get a pretty significant drop, it takes a lot of pressure to release the springs. But that's normal pressure loss, not like you've been experiencing.

My gut instinct says the leak is probably in the supply part of the system and that the air is pushing back out of the primary and secondary tanks through the supply line from the supply tank, probably at the tank fitting(s).

This diagram might help you in your troubleshooting....

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Old 11-29-2016, 06:26 AM   #10
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Did this just start with the new compressor? If so that is where you should start looking.
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