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Air brake systems and diagnostics

doublewide

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May 31, 2015
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Last week I noticed a problem with the air brake system on my 1988 Ford F800. I had been using the truck by itself daily during the week and all systems were working as designed. When I connected my trailer to move my dozer is when the problem became apparent.

Problem; Setting the parking brakes (pulling the yellow dash mounted knob out), results in what sounds like a large volume of air either passing through the dash mounted control valve or exhausting from it. At the same time air exhausts from one of the two quick release valves at the rear of the truck. In addition, the same sound of a large amount of air passing through the tank mounted relay on the trailer can be heard. This air flow slows and eventually stops. During this time, the dash mounted air pressure gauge does not show any change in pressure. If I release the parking brakes, once again, I hear a large amount of air flowing through the control valve, the pressure gauge shows a drop in pressure till the governor reaches the cut in pressure, system pressure builds till the trailer tank is full and then all returns to normal.


ab1.jpg
 

doublewide

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System specifics;

As you can see, this truck does not have a trailer supply control valve in the cab.


ab2.jpg
 

doublewide

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It appears to me that there are two relays at the back of the truck. And each is connected to a quick release valve. One relay provides air to the service brakes and one relay provides air to the spring/parking/emergency brakes. It appears to me that the two relays are connected to each other.


ab3.jpg ab4.jpg ab5.jpg
 

doublewide

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The blue air line on the trailer is connected to the service brake relay (which is relatively new) on the truck and to the “service” port on the tank mounted relay on the trailer. The quick release valve, between the truck service brake relay and the truck service brake air chambers, is where the air exhausts from during this senario. I believe the blue line is the “signal” or “control” line to the trailer relay.

The red air line on the trailer is connected to the spring/parking/emergency brake relay on the truck and to the “emergency” port on the tank mounted relay on the trailer. I believe this is the “supply” line to the trailer relay/tank assembly.

This truck has quick connects at the rear of the vehicle. No glad hands.


ab6.jpg
 

doublewide

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20201021_192733.jpg Diagnosis; Based on my very limited experience, what I believe is happening is the trailer air tank is dumping its contents through the dash valve and the quick release valve when I set the parking brakes. The trailer has service brakes only. No spring brakes. It was my understanding that the relay on the trailer tank was suppose to hold the trailer service brakes “on” when disconnected from the truck, at least until tank pressure is lost. But, considering the problem as described above, it appears that the tank mounted relay may be malfunctioning. This relay and tank are one year old;

Liddell-Birmingham

Any thoughts would be greatly appreciated.
 

workshoprat92

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Dec 15, 2014
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Bois D Arc Missouri
Its probably normal. I have never seen a trailer spring brake Siamesed to the tractor park brake valve. Usually there is a second valve with a red knob. That being said the extra volume of the lines to the trailer being Siamesed in that way would definatly make it sound like its exsausting more air. Well because it is! If it had its own valve and circuit like normal the volume in the trailer circuit would not be so large as it would be combined trailer/ tractor.
 

Birken Vogt

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I don't think you can even have a vehicle to pull an air brake trailer without a red valve in the tractor. And a tractor protection valve is another necessary component.

However, if the trailer reservoir is exhausting through the yellow valve when you set it, then that also indicates a problem in the trailer. The trailer should not release much air through the red line when it is relieved, but rather there is an inversion valve off the trailer air tank that should apply tank air to the trailer cans as you already know, until it leaks away.

So the problem is two fold. The trailer is releasing its air through the red line, and the truck is kind of a cobbled up mess. Curious that it apparently has no proper trailer towing air, except for the hand valve on the dash there.

How well do you understand this diagram? How many of these components are missing?

airbrake.jpg
 

doublewide

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Thanks guys, I really appreciate the input. A little background;

I believe that this truck is a former municipal truck and it looks to be all factory to me but who knows. I have owned it and the trailer for 10 years and never had this problem before. I bought the trailer from a local trailer dealer who replaced all of the brake chambers, a couple of the slack adjusters and the rubber air lines. The truck has been into the local heavy truck service center for inspections and repairs several times and no one has ever suggested there was any problems. I also pass State Commercial Vehicle Inspection every year.

I have been studying my Ford air brake manual that contains similar information to that diagram posted by Birken Vogt.

Agreed about adding the trailer air supply valve and tractor protection valve (if there is none). I wont be back to work till Monday to make additional review of the existing and or missing components.

Thanks again.
 

Birken Vogt

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I suppose it is possible that the truck was that way from the factory. Maybe the requirements were different for a 2 axle bobtail and tag trailer back in 1988. Ford did some strange things back then like the vertical park brake flipper valve in C-series, I always thought they had to be a yellow diamond by then but they still made them that way.

I'd like to see the page in that book that deals with the F-series system.

At first glance it looks like somebody just grabbed the easy connections at the back for trailer brakes but then where does the hand valve in the cab tie in? There has to be another double check somewhere for it. Quick connects don't quite add up either. I would expect everyone was using glad hands back then but they could have been replaced. The ball valves are also suspicious but one of them has possibly original paint on it.
 

suladas

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Thanks guys, I really appreciate the input. A little background;

I believe that this truck is a former municipal truck and it looks to be all factory to me but who knows. I have owned it and the trailer for 10 years and never had this problem before. I bought the trailer from a local trailer dealer who replaced all of the brake chambers, a couple of the slack adjusters and the rubber air lines. The truck has been into the local heavy truck service center for inspections and repairs several times and no one has ever suggested there was any problems. I also pass State Commercial Vehicle Inspection every year.

I have been studying my Ford air brake manual that contains similar information to that diagram posted by Birken Vogt.

Agreed about adding the trailer air supply valve and tractor protection valve (if there is none). I wont be back to work till Monday to make additional review of the existing and or missing components.

Thanks again.


The trailer may be plumbed legally, and the truck may be plumbed legally for itself, but definitely not as far as pulling a trailer on air, so each could technically pass an inspection on their own. Also just because it passed inspection doesn't mean it's legal, here anyway the inspection guidelines are so loose it's not even funny, it's basically up to each shop to decide what it includes and what it doesn't. I've seen so much shotty stuff passed/missed I still inspect my stuff after it's back from the shop to check any work done and to look over the truck/trailer myself because I just don't trust them. I had a truck plumbed for air lines for trailer at a dealership, and they didn't put in a tractor protection valve and just teed a line off the rear axle of the truck for trailer. It's pretty sad but right now there is not a single shop I totally trust. Be a different story getting pulled over and checked, I cannot imagine those quick connects being legal, the way those safety chains are hooked up also not legal here.
 

doublewide

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.....I'd like to see the page in that book that deals with the F-series system......

No F series specific page. All information is general to all trucks.

.....Quick connects don't quite add up either.....

I'm curious about that... Do any of you guys see trucks with quick connects? I'm not around a lot of trucks but have seen one other. A very sharp, customized tractor run by a heavy wrecker outfit from Attleboro, MA area.
 

Truck Shop

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Glad hands are the industry standard for years, I don't believe there is a rule against quick connects. But it is very impractical from the stand point you would have to carry glad hands with
male ends to hook up to other trailers. But the real problem is volume of air through a quick connect like used on your truck. Brake system recovery is all about volume as well as psi.
 

lantraxco

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Ford used a lot of morphodite air systems, that aluminum valve in the picture where the trailer air has a shutoff with a yellow handle may actually be a tractor protection setup. Some of the Louisvilles had it built into the nightmare of a foot valve sandwiched on both sides of the firewall. Don't ask. Just believe that Ford liked to roll their own when everybody else pretty much bought off the shelf. Politely suggest that unless you can find an exact air schematic for your exact truck, don't change anything without a lot of head scratching.

Quick connects are common around here on dump trucks and lowboy (detachable gooseneck) setups at the back of the gooseneck.

Sounds to me like you just have a gummed up check in the trailer relay valve, Just push in the parking brake, to be safe probably with the engine off and in gear so it doesn't try to roll away, then while it's all pressurized just pop the red trailer line off the quick disconnect, see if you actually are emptying the trailer tank there. If you look at the trailer brakes you may see them apply until the pressure drops too low to keep the cans extended. Relay valve failures are very common especially on a setup like this where it's really hard to keep dirt out of the open ends on the lines. YMMV
 

Birken Vogt

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The aluminum valve in that picture appears to be an R-7 spring brake inversion modulating valve. It releases air from the spring brake cans and refills from the front reservoir to apply rear brakes proportionally if you lose air in the rear brake tanks. Feature on straight trucks only, otherwise you would have only the tiny front axle brakes to stop you if you lost rear air.

Agreed on the failed trailer valve being the primary cause of failure and the OP has the correct diagnosis I believe.

Agreed not to change anything until you draw a schematic of the air system with part numbers if you can find them on all the control valves.
 
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