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Old 12-13-2015, 05:19 PM   #1
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Spring Brake Repair

I have a leaking spring brake cylinder. It was easy to diagnose because when I would release the brake you could hear air leaking in the area where the cylinder is located. I got under the coach and disconnected the air line and connected it to shop air and the leak was very apparent. The cylinder is now on the bench.

My question is do you normally replace the cable when making this repair. It seems to move easily. Do you lubricate it or is that a bad idea since it has a plastic liner.

Also before I found the problem when I would release the brake from the drivers seat and heard the air escaping the park brake light would come back on. What turns that indicator light on. I would expect the light to come back on since the brake is not released but I didn't find any sensor or switch near the cylinder or the brake. Did I miss something.
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Old 12-13-2015, 05:47 PM   #2
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Normally, the parking brake chamber leak is just a bad rubber pancake. Be sure to use a release tool to hold the spring before removing the chamber clamp.

Your indicator light is just based on air pressure in the release line. If the leak is not bad enough, pressure will build enough to trigger the light. There isn't any sensor on the actual brake.


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Old 12-13-2015, 07:12 PM   #3
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Be careful messing with spring brake chambers. The spring can knock your teeth out.

Just a failed diaphram is one possibility but I have found a few where the spring has broke and pierced the diaphram. In that case a new chamber is the answer.

Caging the spring is critical if you plan on opening it up.

Some chambers have a crimped band on the spring brake side of the chamber, making the parking diaphram unservicable.

You can replace the service brake diaphram, but again only with the spring caged.

The parking brake light switch is usually behind the dash, right on the control. The switch is designed to pop out as the air drops as a safety measure.

Make sure you adjust up the brakes after putting the chamber back on.
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Old 12-13-2015, 08:57 PM   #4
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If a spring brake chamber is bad it must be replaced.Air Brake Chambers: AnythingTruck.com, Truck & Trailer Parts and Accessories Warehouse
They are not expensive to purchase <$75.
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Old 12-13-2015, 11:01 PM   #5
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IF the unit has been leaking for awhile--guess I would be concerned that the drive shaft brake shoes has been dragging on the drum. Probably need to check the condition of the shoes and drum.....
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Old 12-14-2015, 08:48 AM   #6
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Here are a few tips from someone who was a factory instructor on air braking systems for big trucks and buses.

The foundation brake are operated by the foot brake treadle valve which are directly coupled to the brake air chambers service port , basically the front of the actuator.
These come in various sizes and must not be interchanged.
To test for leaks build the air pressure to max release the air brakes with the wheels chocked on level ground.
Listen for air leaks and observe the secondary tank for a falling needle, any constant leak denotes the spring brake section is leaking either from the inner piston or diaphragm (as applicable) or the center shaft seal.
Since the system is often fitted with a quick release valve it may feasibly to leaking from spring brake system QRV.
Now to add one thing the foot brake system and the secondary park brake system are interconnected but sealed by an anti compound valve. (Often known as a differential protection valve.)
This feature backs off the applied spring brake when the foot brake is applied when he vehicle is parked , thus preventing brake mechanical overload.
Should there be a leak in the anti compound valve air will leak out through the primary air brake system at the QRV or the treadle valve exhaust port.
This symptom can easily be confused with an actuator center shaft seal leak.
To isolate the area simply remove the SERVICE port air connection , release the parking brake and look for air leaking from the actuator at the servcie connection.
Need I mention it but excessive air leakage when the vehicle is in operation; it causes accelerated compressor wear , shortens its life, causes high oil carry over which contaminates the primary air system desiccant drier.
Of course safety is the primary object here to prevent a run away so make sure the rig is adequately chocked before you test.
All the best
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Old 12-14-2015, 09:07 AM   #7
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Thanks for all the replies. Old Scout - It appears it hasn't been leaking for a while. I got under there this morning and can get a peek of two of the shoes from the back of the drum and they have close to a 1/4" of shoe material left.
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Old 12-16-2015, 08:02 PM   #8
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Quote:
Originally Posted by Ray,IN View Post
If a spring brake chamber is bad it must be replaced.Air Brake Chambers: AnythingTruck.com, Truck & Trailer Parts and Accessories Warehouse
They are not expensive to purchase <$75.
Take this advice and go get new or rebuilt brake can. You cage these things to get them off of the road or to the shop to be replaced.

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Old 12-17-2015, 07:49 AM   #9
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I did decide to buy a new one. I am confident I could have disassembled the actuator safely but if I consider the time and expense of fabricating a tool to do it safely and the possibility that maybe there was some kind of internal damage that would require replacement I thought I would bite the bullet and buy new.

After several calls to Haldex and one by my parts guy we found that the original part number is not available. Haldex did cross a new part number and that is what we ordered. It should be here in a week. I will post that part number when I get the new actuator and confirm it is correct.
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