No emergency braking!

[Ronmar]

I don’t have any emergency brakes! You probably don’t have any either…

Emergency brakes: someone came up with a method to use the park brake springs with pedal input in the event you loose your primary/rear brake air. Its Completely automatic, no additional operator workload. its the whole reason to put in that inversion valve in the park circuit.

That inversion valve in the park air circuit compares primary air from the tank and secondary treadle air. It uses this info to control how park air from the dash control is applied to the anti-compound relay to compress the park springs/release the park brakes. Primary tank air should always be greater than secondary air from the pedal, this(and a small spring) holds the inversion valve spool at one end so park air from the dash control can flow right thru the middle of the inversion valve spool, to the anti-compound valve input to compress the springs.

But lets say you blow a hose and have lost primary air. When you step on the pedal, without primary air to keep the inversion spool at one end, the secondary treadle pressure on the other end forces the spool to shift against its small spring. When it shifts, it cuts off supplied park air thru the spool and vents the park air being fed to the anti-compound valve. As park air is vented, the anti-compound valve in turn vents air to the springs causing the springs to start to apply braking force. When you let off the pedal, the spring in the inversion valve pushes the spool back, re-applying park air and releasing the brakes.

Without this, you only have the few pedal presses left in the secondary tank and the single can front axle brakes to safely slow and get this beast to the side of the road. Emergency braking should give you ~50% of the normal braking force to the rear axle.

There is an “emergency” lamp in the dash, but our low bidder friends at S&S neglected to install a pressure switch to the output side of the inversion valve or wire it to that emer light to indicate when the park air is being vented by the inversion valve and this feature is being activated.

So thats how it is supposed to work. Now here is why it wont ever work on my truck(and maybe yours also).

I have written about and done some videos about how the anti-compound valve should work and how to test it. The anti-compound is a relay valve that has 2 inputs thru a built-in 2way check valve. One input is park air to release the springs, the other input is treadle air to also release the springs to prevent damage if you step on the pedal without releasing the park brakes(compound forces).

in looking at the diagram today researching @GeneralDisorder s dryer issue, What finally dawned on my pea brain this morning was the fact that my anti-compound is not plumbed like the diagram in the schematic. In the schematic that treadle source to the anti-compound valve is supposed to be from the Primary side of the pedal, so if I lost primary air, that input would also be missing. But in reality on my truck it is connected to the secondary side of the treadle, in fact it is “T”d off the very line that feeds the inversion valve For emergency braking

So on my truck if I loose primary air, when I step on the pedal, secondary treadle air will be shifting the spool of the inversion valve and venting park air to try and engage the park springs at the anti-compound valve. But at the same time, that very same secondary treadle air will also be supplying the anti-compound valve control input keeping park air applied to the springs and cancelling out emergency spring brake application.

With the line with red X’s connected, emergency braking will never work!

instead the line should be like the line I drew in blue coming from a primary treadle source.

So far the only place I see the actual connection specified is in the schematic diagram, and that shows primary treadle output feeding the anti-compound. The replacement procedure for those valves doesn’t even show the line functions or destinations, they simply say to mark everything well and copy the fitting orientations during replacement, and of course no procedure to actually confirm it is working

S&S strikes again, and it only took me how many years to notice it i must be getting old…

 

[GeneralDisorder]

It would appear that my A1R is plumbed how you are suggesting. I don't have the load valve but my anti-comp valve is T'd into the primary treadle valve supply for the relay valve below it and I have no secondary treadle valve supply T at the brake protection valve.

 

[MatthewWBailey]

Is it possible that these air lines can get "redone" wrongly by "unit maintenance", but never discovered?

 

[canadacountry]

@MatthewWBailey considering that its only for emergency purpose relatively speaking I guess that I won't be surprised if perhaps indeed noone knew it was put back on wrong till the one day someone does need it and it doesn't work

 

[GeneralDisorder]

Is it possible that these air lines can get "redone" wrongly by "unit maintenance", but never discovered?

It probably would never happen, and the reason is simple - the vast majority of the time you are either replacing a single part at a time, or replacing one line at a time - part fails or line fails, etc. And in the military if you forget what went where you just go look at another truck in the yard. And these are largely one-time-use lines. In that if they fail you usually just make a new one if you can't just change the end and have it reach.

It could happen but I kinda doubt it. I'm betting it was configured this way from the factory.

 

[Ronmar]

Is it possible that these air lines can get "redone" wrongly by "unit maintenance", but never discovered?

I suppose anything is possible but it would require elbows placed where Ts are supposed to be and vis versa not to mention the line lengths might not work out for re-attaching existing lines. It would take some extra effort to re-plumb, especially like General mentioned, usually one piece direct swap at a time…

 

[Guyfang]

Why don't you get 4-5 people to take a look see. The last year I worked as a FMT, Field Maintenance Tech, I spent the whole time driving around Germany, fixing Brand New Trucks, that had been assembled wrong at the factory. Or teams of civilians going from unit to unit "Upgrading" equipment, made a lot of mistakes. This is not something that happens once in a blue moon. The first HEMMT A4's to be delivered to Germany, 104 of them, we had to inspect all of them for defective Transfers. From the factory. Our team changed about 40-45 of them. Another team changed about the same number.

This statement kinda rubs me wrong: "Is it possible that these air lines can get "redone" wrongly by "unit maintenance", but never discovered?"

I am sure it could and probably does happen. But I know for a fact, that from 2004-2010 when I stopped working as a FMT, 95% of all maintenance on Vehicles done in Germany, was done by civilian contractor maintenance. And some of that work was very crappy. Not all, but lots of the "Mechanics" had lots less training than the Soldiers. I could be wrong, but don't think it was much different in the States.

 

[Ronmar]

Yea like the wet tank plumbing so far, I suspect this will probably turn out to be an A0 thing. Unless you are specifically looking for it, It is up under the bed and not easy to see Unless the bed is off. i have looked at all that plumbing perhaps a hundred times, but it took having my head in the right place for that squirrel to catch my attention… i will ask in the facebook group to try and get a few more data points.

 

[Guyfang]

Last week, I was renaming PDF files, TM's and such. I found a MWO, Modification Work Order, that was a rework for a brake system on another truck. From the factory the system was installed wrong, and a team of TACOM people went around to "fix" it. And they in fact made it worse. So the MWO came out to Fix it yet again. A sad situation.

 

[Ronmar]

Last week, I was renaming PDF files, TM's and such. I found a MWO, Modification Work Order, that was a rework for a brake system on another truck. From the factory the system was installed wrong, and a team of TACOM people went around to "fix" it. And they in fact made it worse. So the MWO came out to Fix it yet again. A sad situation.

Yea they were sometimes called ”Tiger” teams in my service. We used to have to go “tiger hunting” after they left

 

[MatthewWBailey]

Yea like the wet tank plumbing so far, I suspect this will probably turn out to be an A0 thing. Unless you are specifically looking for it, It is up under the bed and not easy to see Unless the bed is off. i have looked at all that plumbing perhaps a hundred times, but it took having my head in the right place for that squirrel to catch my attention… i will ask in the facebook group to try and get a few more data points.

Which fb group?

 

[Ronmar]

Which fb group?

lmtv mtv military trucks

 

[Third From Texas]

It would appear that my A1R is plumbed how you are suggesting. I don't have the load valve but my anti-comp valve is T'd into the primary treadle valve supply for the relay valve below it and I have no secondary treadle valve supply T at the brake protection valve.

View attachment 925463

Curiosity made me look and my 2008 A1R. It's plumbed like GD's as expected.

 

[Ronmar]

TO TEST IF YOUR EMERGENCY BRAKING INVERSION VALVE/SYSTEM IS WORKING:

This will work better with two people.
On level ground, chock the wheels, start the truck and fill the air tanks(dryer purges).
Apply park air/release the parking brakes and shutdown the engine.
Open the tank drain and release all the air from the primary(middle) brake tank.
With someone listening near the drivers rear wheel, or under the truck to see/touch the inversion valve(drivers side of relay valves) over the rear axle, depress the pedal.

With both primary and secondary air available, normal brake application/operations should only ever VENT air as you are RELEASING the pedal...

With primary air gone the inversion valve should vent park control air from its vent port on the valve and the anti-compound calve should vent park air from the actuators from its bottom vent, AS YOU PRESS the brake pedal.

This is the only time you should hear any air venting to atmosphere from a brake system while you are PUSHING on the brake pedal.

I suppose if you had a safe place to drive, you could drain the primary air, mute the low air buzzer and see what the emergency brake response feels like while rolling. I suspect it may be a little harsh I suppose it depends on how fast the inversion valve and anti-compound vent the park air/apply the coil springs, and how much weight you have on the back.

 

[aw113sgte]

In theory, should I be able to open the drain on either primary or secondary tanks and still have an operable truck?

 

[Ronmar]

In theory, should I be able to open the drain on either primary or secondary tanks and still have an operable truck?

Partially operable… you will loose that system that is fed by the tank you drained. Federal motor vehicle safety standards(FMVSS) dictates that the systems be separated. They are fed with a common source(compressor, dryer, wet tank) but thru individual check valves on the service tank inputs, just like your car primary and secondary hydraulic brake systems are fed from a common reservoir, but to separate pistons in the master cylinder.

If you drain front/secondary service tank, you loose the front/secondary brake system. Drain the rear/primary service tank, you loose the rear service brakes, but the emergency system should apply the park brakes(~50% normal force) in their place on the rear until you are out of secondary air.

Park air that holds the park springs compressed to release the park brakes is derived by a 2way check-valve that samples both the primary and secondary tanks. drain both primary and secondary air, the park springs will start to engage as soon as the highest tank pressure starts to pass below ~60PSI.