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Old 02-27-2021, 08:17 PM   #1
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Low air

Hello lovely people!

Our bus loses a lot of air overnight resulting in the bus being very low to the ground after sitting just a day or two. How normal is this? Should we be concerned about an air leak?

Thank you!

MandM
2002 bluebird tc 2000
Cummins
25ft flat nose
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Old 02-27-2021, 08:43 PM   #2
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It is normal. Mine leaks down faster in winter than summer. If it leaks down fast, or you can hear a leak, then you have a problem. Someone here may know the DOT limit to how past your PSI can drop before it is a "problem"
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Old 02-27-2021, 09:01 PM   #3
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Quote:
Originally Posted by MandM View Post
Hello lovely people!

Our bus loses a lot of air overnight resulting in the bus being very low to the ground after sitting just a day or two. How normal is this? Should we be concerned about an air leak?
I don't care what anyone says, an air leak of any kind is not normal if air pressure bottoms out overnight in any part of the system. I know some folks regard this as normal on an older vehicle, but it indicates a problem that needs to be fixed and is, in fact a safety issue (See my thread "Safety Inspection Checklist", linked with others below in my sig).

The reason an air leak is not to be taken lightly is that a small air leak can not only become larger over time, depending on what is leaking, it can cause a sudden major air loss, which will lock up your parking brake almost immediately, a dangerous situation if it happens while driving, it can cause complete loss of control, and even mechanical damage.

I have seen more than one member here find themselves stuck in the middle of nowhere. One was in Joshua tree with no immediate help available when it happened to them. Get it fixed. You may be very sorry you didn't if you don't. There are no unnecessary parts on these things.

The DOT limit, BTW, is no more than 3 psi loss in one minute with the engine off, sounds like you have a bit more than that.
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Old 02-27-2021, 10:16 PM   #4
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If you lose 1 psi every minute, which is considerably less than the limit, you'd go from 120 psi to 0 in...drum roll...120 minutes....two hours. So, this probably isn't an issue and our buses all drop after a day or two.

Keep in mind that the air bags should be on a separate tank from the brake systems, separated by a pressure protection valve and a check valve. So they'll typically keep air pressure longer than the brake systems. If you only had air bags on that aux tank, they'd probably stay up for days...or forever. We have one vehicle with airbags and we separated them from the tank, so each airbag has its own schrader valve. Those keep air pressure for many months. But you've also got other things on that aux tank...the air ride seat, door actuators, who knows...and those may leak.

If you need the airbags to remain inflated...especially if you want to be able to adjust level when camped...you might consider separating them from the aux tank and putting each airbag on its own line. You can be super simple, with just a schrader valve you'd use a tire air chuck to inflate. Or you can put in electric valves so you can adjust them with the push of a button. Just don't connect both airbags to the same line, with a tee, as that can cause some uncomfortable sway as air moves from bag to bag in turns.
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Old 02-28-2021, 12:52 AM   #5
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I agree with Cheesewagon, on a bus the air system is a critical system. Even if your air bags are on a separate tank, you don't want one of the bags to suddenly deflate going 60+ down the road.

You especially don't want your emergency brakes suddenly fully engaging, or worse, if the leak is in the reserve tank, the emergency brake not working!

It all simply goes from bad to worse.

I do like the separate air bag tank idea and individual Schrader valves, especially if you have a heavy load of batteries, inverter/charger, etc. all on one side.

A good way to test is to get a spray bottle with soapy water, build up the air pressure to as high as it will go and start spraying and watching for bubbles, as well as listening. You might also consider a really good inspection of all your air hoses and connections.

If you are going to replace the hoses, unless you have the right equipment to crimp connections, I'd recommend having it professionally done. It doesn't matter if a hose bursts or comes off a connection, it all ends up the same way....no air!

Stay safe.
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Old 02-28-2021, 06:23 AM   #6
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The air system is critical...that's true...and I greatly respect those who are urging caution here. This system isn't something to take lightly. But I also don't want to instill fear.

The lines to an air spring (air bag) are small enough that if one bursts you won't suddenly end up with locked brakes. If the compressor can't keep up with the air loss when this happens, your air pressure will drop from normal (110-120) and somewhere around 60 psi your warning light and buzzer will come on and somewhere around 20-30 psi the rear brakes will come on. But in my practical experience, having had an old airbag tear and flatten completely, the air compressor puts out so much air that this takes quite a while. In fact, on that particular bus, I was able to continue my trip...while constantly watching the air pressure...and the system kept up just fine. The bus leaned to the left, but the air system kept up. I don't recommend this, of course, but I also don't want the OP to worry they'll be driving down the road and boom lock up the brakes without warning.
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Old 02-28-2021, 07:12 AM   #7
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Quote:
Originally Posted by MandM View Post
Hello lovely people!

Our bus loses a lot of air overnight resulting in the bus being very low to the ground after sitting just a day or two. How normal is this? Should we be concerned about an air leak?

Thank you!

MandM
2002 bluebird tc 2000
Cummins
25ft flat nose
Here is what I would do...
Get a spray bottle with a soapy water mix and a small paint brush.
Start your bus and wait till air tanks are full then turn motor off.

I myself would start at the rear wheels and work my way forward.
Spray the soapy water on your air bags and all fittings and hoses that you can see, using the paint brush when ever needed to move any dirt and grime.

When you are spaying the soapy water, if an air leak is found you should see bubbles forming. Small leak, small bubbles, large leak, large bubbles.

Spray the air lines all along the frame rail working you way forward.
don't forget to spray the brake pedal assemble and also the air lines that go to the dash gauge too.

Like Rossvtaylor said, if it drops in 2 hours or less then you have an issue that needs immediate attention.

If it takes longer than 2 hours to drop, say 8-16 hours for example, you may have a pinhole leak, bad seal or check valve somewhere and the rear air bags, they do develop cracks in the rubber so insect them well.

Almost ALL air brake equipment will loose at least 60-80% of air in their tanks over a 24 hour period but what you want to know, specifically, is what is the loss rate of air in YOUR bus!

Air lines and fitting are easy to replace and so are the air bags so don't fear the job, it is quite easy to do.


BTW, air compressor check valves is a common place for air leakage to occur. They get dirty with oil if your air compressor is old and tired.
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Old 02-28-2021, 07:27 AM   #8
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From a safety design perspective, keep in mind that the aux tank is (or should be!) separated from the primary/secondary brake tanks with the pressure protection valve I mentioned in my earlier post here. Its specific purpose is to prevent brake system failure in the event you lose pressure in your aux system. On many buses, you'll notice that when you start the engine the air pressure builds to about 60 psi...then sort of stalls out for a while...then builds pretty quickly up to full pressure. This is because the compressor fills the primary and secondary brake tanks first, then above about 60 psi air is allowed into the aux system to fill that tank and the air bags and the seat. Once those are pressurized, the system fills to full pressure. In theory, even if you tore a hole in your aux tank your brake system pressure wouldn't drop below 60 psi...this would cause a low air warning, but your brakes would still work to get you stopped. It's really a very safe system.

All of this applies to modern systems, of course. On the old 1949 GM 4509 we have, and the recently-purchased 1956 GM 4104, if you lose air pressure you lose all braking besides the hand-operated driveline brake. Wheeee!
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Old 02-28-2021, 08:36 AM   #9
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What ewo1 just stated use soap and water and look. I have had a CDL since before they were called CDL’s almost 50 years. I was also a CA licensed class A lamp, brake and smog inspector. If a truck got a ticket for out of compliance brakes someone like me would have to put it back to spec. I used a Hudson sprayer and dawn dish soap to look for air leaks. Most of the time it was the drain valves or the new at the time water ejectors. These things receive all the water and other crap in the system and cause the valves to not close properly. When air ride became more popular the level valves became the main leak points. It is a valve that takes a lot of abuse bouncing down the road. One word of caution that I didn’t see mentioned is if it has air ride DO NOT get under it unless you have safety stands under the frame. If that bus falls on you your loved ones will know the meaning of “thrown under the bus” you will be flatter than a girl scout cookie in a walmart parking lot.
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Old 02-28-2021, 10:53 AM   #10
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the air brakes on my c7500 stay up for days in the summer, drop in a day in the cold weather, I suspect the nylon air lines are shrinking in the cold. O rings are generally good for only @ 20 years before shrinkage becomes a problem. I feel sorry for all these contractors using shark bite fittings on plumbing, it will become a problem later in life.
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Old 02-28-2021, 02:13 PM   #11
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amen on the shark bite fittings.
i only use them on test rigs. and not to many temporary fixes.
the other i have not used and want to see a history log of is the pro press stuff.
neither are allowed on the govt installations where i work so i have only seen it used. and have always questioned it?
i know even with test rigs on copper the shark bit coupling need to be supported perfectly straight but then again i dont use pex piping much either.
my first bus is hard pipe copper and my second bus is getting ready to be as well.
the one i am questioning is the soft copper provided with my mini split but if i can leave a little bit of coil at each connection point it should be fine until the actual connections eventually work harden over time.
i am thinking of having rubber hydraulic hoses made with the correct flare ends and protect the hoses at the bus body penetrations and never think about it again.
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