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Old 12-14-2006, 09:54 PM   #1
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Backup 12v Air Compressor

Has anyone ever connected a backup air compressor to an existing air brake system? I started up my bus again today and the air horn was going off again. I ran it for over 5 minutes at 2500 rpm so I think I still have a leak and the horn never went off. I fixed one but it still is leaking. So I need to find out where the other leaks are. I went ahead and purchased a 12v compressor that has a 100% duty cycle at 100 psi that I am going to use to trouble shoot the system. I will probably use the system to also inflate tires and other things once I install it. However, I was also thinking this could be a good backup system should the engine mounted compressor ever fail.

So my question is – has anyone ever performed an installation like this?

If I ever hook it up I would make sure it has an aux tank and a one way valve plus an overpressure valve for the tank.

Another advantage of having another tank would be that it would be easier to inflate any tires that need inflating plus it should be able to operate some air tools.

So if anyone ever hooked an emergency backup air compressor I would like to hear from you.

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Old 12-14-2006, 10:00 PM   #2
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I never hooked one up as a backup air compressor but I do know they're used a lot by the over-the-road bus conversion folks. They have air suspension and brakes and quite a few install air compressors to air up the bus in the morning rather than running the diesel for 10 minutes or so. I think in an emergeny if would let you move some but I'd be worried about volume of air rather then pressure if you use the brakes much.

Interesting idea though!
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Old 12-15-2006, 10:45 PM   #3
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I asked our service people about doing this a while back, they told me that it is not legal...I guess as a back-up it certainly couldn't hurt. You really just need to find your leak. I can park mine for weeks at a time and it doesn't loose any air pressure at all....I can fire it up and it'll still be reading 100 psi on both tanks!

You just might be a Redneck if...
...your motor home used to be a school bus!
...Your living room has a steering wheel!
...Your home has brake lights

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Old 12-16-2006, 01:57 AM   #4
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As was mentioned, the volume of air is an issue. All of the 12V compressors I have seen don't move a lot of volume. This works for airing up the suspension because it isn't time critical. But air brakes use a lot of air and can require it frequently. As a cautionary example, in situations where frequent braking is required, even the engine driven compressor sometimes can't keep up and the brakes grow weaker.

As an emergency backup, I suppose it is better than nothing, as long as you don't allow it to make you complacent about repairing problems with the main system. Starting with fully pressurized (120psi) primary/secondary tanks, you will only get a few applications of the service brakes before you drop to 80psi and need to pull over to recharge the air tanks for 10-15 minutes.

12R22.5 tires I've seen run at 95-110psi and take a fair volume of air just to nudge up the pressure 5 pounds. Because of that I'm not sure using a portable tank to add air to bus tires would work. A long air hose would be best. A long power cord would work for a while, but the voltage loss over a long cord would soon burn out the compressor motor.

Most common air tools require consistant high volume and pressure. For example:
  • > Campbell Hausfeld 3/4" Impact Wrench - 6.1cfm @ 90psi
    > Ingersoll Rand 1" Impact Wrench - 9.5cfm @ 90psi
    > Ingersoll Rand 4-1/2" Angle Grinder - 9.0cfm @ 90psi
    > Campbell Hausfeld General Purpose Spray Gun - 4.3cfm @ 40psi
    > Campbell Hausfeld HVLP Siphon-Feed Spray Gun - 9.0cfm @ 40psi

At a google glance, here are some typical 12-volt, continuous duty compressor's high pressure cfm ratings:
  • > ExtremeAire - 1.5cfm @ 100psi
    > Thomas TA4101 - 0.95cfm @ 100psi
    > Viair 450H - .94cfm @ 100psi

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Old 12-16-2006, 11:56 AM   #5
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So does anyone have a rough estimate of what the output of our air brake compressors is then? I'd like to know so I can estimate how big of an auxilary tank I would need to hook up (when stationary only of course) to get any sort of run time before the compressor can't keep up anymore for just basic tools like an impact wrench or an air cannon for music festivals :P
Skooling state at a time...
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Old 12-16-2006, 12:28 PM   #6
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Air volumns

Each air brake system is designed for a specific application. Each vehicle maker has the engine supplier, supply the compressor required.
Air compressors are supplied that can supply from 8-18 cubic feet of air per minute. All compressors will pump up to 120-150 PSI, based on the pressure switch/govenor. The design of the air compressor is very complex with many requirements to be met. It is not possible to interchange a compressor from any application to any other application. In many cases, the compressors are not able to be attached.
In some cases the long /tandem buses, have more air brake diaphrams and require more air in storage tanks. My Crown school bus has 5 small air storage tanks. It would not be easy or convenient to move a bus using only a small electric compressor. It could be done with time and patience, but could create many problems on an large hiway. I have moved large trucks and buses, usings a gasoline powered air compressor. When the air pressure is lowered to about 70 psi, the vehicle stops and cannot be moved until the air pressure is raised to over 70 psi. The air pressure controls stopping and moving. On several occasions, I have moved a bus with a bad compressor or engine by connectind the front truck/bus to the rear truck/bus with an air line.
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Old 12-16-2006, 03:44 PM   #7
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Ken, please take any leakage in your air brake system very seriously. It should be a matter of pride to have it hold air for several days.
A bonus to a leak free system is that you will know right away if a leak develops.

Says the guy with 2 million safe miles on air brakes.

Oh, and Merry Christmas to you and yours!
Millicent The Bus - roof raised two feet, toy-hauler tailgate.
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Old 12-16-2006, 08:03 PM   #8
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i was under the impression that the majority of the air brake compressors were supplied by some one like bendix, and that the various drives are set up to fit the engine, check with a truck parts house or truck brake specialist.
I use a belt drive york v-2 ac compressor off of a 80's dodge pickup for air on my homebilt airride jeep thing
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Old 12-16-2006, 09:08 PM   #9
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Like Frank said, it varies from bus to bus. But using an example I am familiar with, buses that have Detroit Diesel 6V or 8V series engines often use a gear drive Bendix compressor rated at 15.9cfm.
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Old 12-16-2006, 09:43 PM   #10
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I think everyone agrees that brakes are the most important area to maintain in 100% working order. But sometimes it is easy to focus ourselves elsewhere while converting a bus while believing that the brakes will be "ok for now". It is also critical to be careful modifying the air system as a mistake could reduce braking power.

In the tragic bus accident linked to below the driver lost both of his legs above the knees and was lucky to survive. I haven't seen anything on why Daniel didn't stop in time so it may have been brakes or it may have been fatigue. But this is exactly what can happen to anyone who takes one of these heavy beasts out on the highway with insufficient braking power.

(Note: For anyone interested, a donation site was set up by a respected member of the MAK bus conversion forum to help Daniel with his medical expenses. It can be found at )
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