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Old 04-19-2013, 02:43 PM   #1
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Motor Home air brakes

I believed brakes were released by the application of air pressure and if you have do not have sufficient air pressure then your brakes will engage. I believe that is correct for parking brakes. Where I am not sure and there seems to be a difference is in your normal brake operation.

This website seems to support my idea :
HowStuffWorks "Understanding Brakes"

However a website posted by someone that was trying to help me seems to show a totally different approach.


does anyone know which is correct for most motor homes and specifically for allegro bus.
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Old 04-19-2013, 03:30 PM   #2
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All air brakes in automotive applications, including motor homes, operate in a "fail safe mode." Air pressure is required to releace the brake. Low air pressure, and the rig ann't be driven.

I had it happon on an American Eagle, and we on the side of the road for half a day waiting for the mobile service guy.
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Old 04-19-2013, 03:50 PM   #3
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The parking brakes are spring loaded. To release them requires air pressure to pull the spring brakes open. Iff you lose air pressure the spring brakes lock up. To move in an emergency, they can be released by using a caging bolt to cage the springs. However, then you will have no brakes at all.
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Old 04-19-2013, 03:52 PM   #4
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Gemin5362, good question. Think of your air brake4s as two (2) systems, parking and service. Parking requires the use of a different type of can. It is a dual chamber device that has two pancakes in it and large (and large spring inside). Air pressure above 25 to 35 psi prevents this spring to recoil. When the pressure drops below that point the spring forces the internal piston in the can to apply the brake. Note: The dual pancake chambers are only used for parking.

Next: On all other air brake systems the activation of the piston or rod that is connected to the brake cam is controlled by air pressure. The more pressure applied via the brake pedal, the more force is exerted on the movable parts of the system.
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Old 04-19-2013, 04:04 PM   #5
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I just reread my post, sorry for the poor spelling. I hope you all understand what I meant to convey.
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Old 04-19-2013, 04:26 PM   #6
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Coaster1 has it exactly right.
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Old 04-19-2013, 04:53 PM   #7
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Quote:
Originally Posted by Coaster1 View Post
Gemin5362, good question. Think of your air brake4s as two (2) systems, parking and service. Parking requires the use of a different type of can. It is a dual chamber device that has two pancakes in it and large (and large spring inside). Air pressure above 25 to 35 psi prevents this spring to recoil. When the pressure drops below that point the spring forces the internal piston in the can to apply the brake. Note: The dual pancake chambers are only used for parking.

Next: On all other air brake systems the activation of the piston or rod that is connected to the brake cam is controlled by air pressure. The more pressure applied via the brake pedal, the more force is exerted on the movable parts of the system.
coaster one I have a generic air brake manual someone else sent to me and it agrees with what you are saying. I also read the link I have in my first post and it says something different. The link seems to say that the service brake and the parking brake work in the same manner. Is the link wrong or is that type of braking system not used on RV braking systems.
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Old 04-19-2013, 05:09 PM   #8
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I must confess to not reading the entire narrative contained in the link you referenced. My explanation, poor as it was based entirely on my 45 years experience in operating tractor trailer combinations in a coast to coast operation.

If you can, take a look at the brake cans on the rear axle, you will note that there are two air lines, one one the top, then a clamp and one on the bottom. You may also see a plug on top of the can. Removing this plug allows you access to a nut which will retract the plunger inside the can and render that can useless.

By comparison, take a look at the cans on the front. A single air line provides pressure via the foot brake from the air tank to the front brakes and the bottom portion of the can on the rear brake cans.

I wish I was more adept at technical writing but, this is the best I can do.
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Old 04-19-2013, 05:55 PM   #9
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The brake canister has 2 ways to work the brakes. The outer most housing has a big spring in it and it sets the parking brake when no air is supplied to it, this will happen if you have less than 50 or 60 pounds of air. The part closest to the mounting has a pancake in it that when the parking brake is released and you step on the brake it applies the service brake.Worked on big truck brakes forever, never try to mess with the side with the spring as it applies about 1200 pounds of pressure when the parking brake is set.Just keep them adjusted good and you will be fine, I don't care if they have auto slacks they still need to be adjusted on occasion. Hope this doesn't confuse you to much but to me it's simple as I work on them all the time. Jim
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Old 04-19-2013, 06:36 PM   #10
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Old 04-19-2013, 06:51 PM   #11
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Pedal goes down and I have to pull them up

Brakes do work any suggestions?
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Old 04-19-2013, 07:18 PM   #12
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Brakes do work any suggestions?
Are you talking air brake? What kind of pedal? Treadle valve on floor or a pedal from under the dash? If it's a treadle valve on the floor it can be a seized up pivot. Also if the air compressor is pumping oil it can get throughout you air system and swell up critical seals.
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Old 04-19-2013, 07:41 PM   #13
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Good question, important to understand air brakes...
Your link to "How stuff works" is wrong. Coaster1 is correct. If you want the long version, the Canadian Air Brake manual, section three goes into detail.
http://www.mpi.mb.ca/PDFs/AirBrakeMa...alComplete.pdf

Canadian drivers need an air brake endorsement on their license, OTR truckers also need to know the operation of air brakes for their CDL. Although not required for U.S. operation of RV's, a good understanding of Air Brake operation and required checks is very important. Especially if you hate surprises.
Trailerkings third link is the air brake test.
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