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Air brake trouble shooting.


Nwcid
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I have about 4000 miles on my new to me 2014 VNL 630 with 420k miles. 

Obviously the "best" answer is take it to the shop, but if it is something I can trouble shoot at home I would like to start there.

Occasionally the "parking" brake will "release" allowing the truck to roll about an inch or two and then locking up again.  I have had this happen enough times not to be a fluke, but not constantly either, maybe 6-8 times. 

Here is what I have found so far:

Has never happened when the truck is parked and shut down.  

The only time it will happen (if it happens) is if the truck is parked, shut off and then started up shortly after, maybe under 15 min (never really timed it).  

Typically within 1-2 min of restarting the truck it will make the "air pressure" bleed off noise as the tanks hit pressure, the truck will roll a little (even on flat ground) and then lock up again. 

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When you set the park valve (pull the yellow control) where does the air exhaust from the brake chamber? It should exhaust near the axles - not inside the cab.

With the park brakes set and the engine shut down, can you hear air moving in the system?

Kind of a mystery - if the brakes are adjusted correctly the only way the park brakes should release is when sufficient air is supplied to the spring chambers to release them.

The only way brakes that are set would release, then set again is if air is bleeding into the chambers, then bleeding off again allowing the spring brakes (parking brakes) to set again. 
In any case this is dangerous. Suggest chocking the wheels whenever you park the truck until you get this figgered out.

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When I set the brakes I can hear air move at the dash, but the exhaust is at the axles.

With the engine shut down there is some air bleed from the fan clutch and something near the rear the axles that lasts for a few seconds. 

There is no problem with them releasing while the truck if off.  Or if I restart it after sitting for several hours/overnight. 

The only time it happens, if it happens, is truck has been running, got shut off for a short period of time, and then started back up.  Then if its going to happen it will be in the first minute or so.

 

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The only connections to the brakes is the yellow knob. If the knob's out, there's no supply to the brakes. No supply, parking brake is applied. There's no ignition connection, no timer of any sort. Confirm proper brake adjustment, including Slack adjustment and S Cam movement. Chock the tires until you get this sorted out.

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 This truck will have anti-lock brakes.?

 So there will be an electric connection between the brakes and likely the ecm, or an electrically charged anti-lock system. If it were my truck, I would attempt to locate the anti-lock system and inspect all wire connection and routings. I am not a truck mechanic, but I have to wonder since this appears to only do this upon a re-start, perhaps some sort of anti-lock feedback or wiring component holding a bit of electric charge might be the culprit.?

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59 minutes ago, Deezl Smoke said:

 This truck will have anti-lock brakes.?

 So there will be an electric connection between the brakes and likely the ecm, or an electrically charged anti-lock system. If it were my truck, I would attempt to locate the anti-lock system and inspect all wire connection and routings. I am not a truck mechanic, but I have to wonder since this appears to only do this upon a re-start, perhaps some sort of anti-lock feedback or wiring component holding a bit of electric charge might be the culprit.?

That's a good thought, but I believe the anti lock only works on the service brake side.  If the park brake is activated a spring sets the brakes.  I have never ran across anything like what is described in all the yrs of working on them.  

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Yes this will be hard to duplicate.

Yes I have ABS.  

In theory they can start by checking the braking system and making sure everything is adjusted correctly.  This truck does have "self adjusting" brakes, but still. 

While the weather was still below freezing I had my drivers, front drive brake routinely be frozen after sitting and I would have to tap on it to release.  

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2 hours ago, dennisvr said:

That's a good thought, but I believe the anti lock only works on the service brake side.  If the park brake is activated a spring sets the brakes.  I have never ran across anything like what is described in all the yrs of working on them.  

 It does. However, the anti-lock "computer" has to know that the park brake is released, so there is a connection there of some type. Upon re-start, should the ecm or separate anti-lock device need to run some sort of self-diagnostic, I can't help but wonder if said device would momentarily glitch and begin release thinking that the wheel sensors are showing movement of some kind? Since this only seems to happen upon warm re-start, perhaps some component is holding a charge or some sensor is faulty and sending a false reading. The ecm knows the engine temp at all times.

 I dont know. I'm just grasping at straws wondering why this happens upon a warm restart and so far anyway, not on cold startup. I know recalls happen on most all new vehicles now days. Perhaps a recall history would also be in order?

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1 hour ago, Parrformance said:

I had this issue with a brake that would not release.  I changed out the chamber and cured the issue.

Would not release all of the time or just in freezing weather?  All it takes to release it is a simple tap with a hammer on the edge of the drum.

I just did 3000 miles with no releasing trouble. 

Edited by Nwcid
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3 hours ago, Nwcid said:

Would not release all of the time or just in freezing weather?  All it takes to release it is a simple tap with a hammer on the edge of the drum.

I just did 3000 miles with no releasing trouble. 

With mine it stuck when sitting for extended period of time. 

It was leaking internally.  This made it take forever to release the park brake.

I just did a piggy back replacement.  Took about 45 minutes the first time. Now I can do it much faster.

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Either your dash valve is leaking or the relay valve that feeds the airlines to the brake cans may have an issue.  

I have antilock on the rear axle of our old Volvo, but I've never had an issue with the truck rolling with the park brake set.  I'd chock the wheels, and release the brakes and carefully crawl under the back and check the pad clearance.  I had 1 automatic adjuster that didn't adjust all the time. 

 

 

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My brake sticking only happened in freezing weather and usually when it had been wet.  My understanding is this is a semi common problem.

I do not think I have any leaks.  I have never timed it or check my pressure levels while sitting.  Even after sitting for a few hours there is usually enough pressure in the system my "alarm" does not go off when starting the truck.  How long should the truck hold pressure?

I thought about the suspension also because I can get it to "roll" if dumping or inflating.  My suspension stays inflated (or mostly) even after sitting for long periods. 

 

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If you've got stability control it'll apply the service side momentarily during the chuff test just after key on.  Leaking diaphragm, bad QR-1C, messed up plumbing from converting it to straight truck, etc, could cause crossover and momentary park release.  Probably need to unhook some emergency side hoses to isolate and see just which ones are blowing air at that time.

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  • 4 weeks later...

Check the back-flow prevention/directional air valve on each air storage tank for leaking. Sounds like the secondary tank is back-flowing slowly into the main tank when moisture in the valve freezes.

I'm assuming you have replaced the air dryer filter recently and drained both air tanks???? Even so you can still get moisture in the tanks and valves. The only way to remove residual moisture is to introduce air-drying alcohol into the system; HDT's used to have a reservoir specifically made for this.

If there is no moisture in the system perhaps the directional air valves need replacing.

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