M931 Trailer air brakes

[D6c]

Hooked my M931 up to my new-to-me lowboy to see how it's going to work.
Maybe I'm doing something wrong, but I can't seem to get any air to the trailer. First thing I thought was that the valves were off at the supply hoses, but it wasn't the case.

The air button below the dash to charge the trailer air tank doesn't want to stay in even if I hold it in for a bit. Got it to hold in once or twice but not for long.

With the trailer air lines unhooked I pulled the manual trailer valve on the steering column. I thought I should have gotten a blast of air out the emergency hose but nothing happens.

Both the service & emergency air gauges show about 125 psi and the truck brakes work ok.

Can anyone point me in the right direction to diagnose the problem?....or tell me what I'm doing wrong.

Thanks

 

[NDT]

Tell us what position the levers are on the air line valves at the pogo stick.

 

[topo]

When hooking up to new to me trailers I have hooked up the lines the wrong way and had to unhook drain the air tanks on the trailer completely to reset the valve .then try again .

 

[EO2NMCB]

There are valves on the air lines by the cat walk that must be opened.

 

[painter paul]

Some older mil vehicles have the shutoff valves the opposite of civilian ones. On a civilian vehicle if the handle on a shutoff valve is aligned 90° to the line it is shut off.

 

[Scrounger]

First off the gauges are primary and secondary systems. And the trolley valve, that’s the name for the hand valve on the steering column sends air to the service side to apply the trailers brakes.
Make sure the airlines are connected properly red, emergency line to red. And blue, service line to blue. Open the valves on the catwalk, then push in the valve on the lower part of the dash. It generally won’t stay in unless there is a mostly charged air system. If there is a question as to which way the valves on the catwalk are oriented, if you can have soldier A hold in the dash valve when soldier B opens the catwalk valve to check for air flow.

 

[73m819]

All so look for mud dobber houses in the air lines.

 

[D6c]

When hooking up to new to me trailers I have hooked up the lines the wrong way and had to unhook drain the air tanks on the trailer completely to reset the valve .then try again .

Swapped the trailer hoses and it locked the trailer brakes until I bled off the trailer air tank so they must have been right the in the first place....swapped the hoses back.

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The valves do seem to operate backwards (lever 90° to line is open)

If I close the valves (lever in line with hose) and push in the dash valve it stays in immediately and the hand "trolley" valve seems to have air.

If I then open the valves the dash button pops out, which makes sense because of the pressure drop. However, no matter how long I hold the button in to charge the trailer tank, it won't stay in.
I'm beginning to think there might be a problem with the tank valve on the trailer.

Note: This is an older trailer that doesn't have self-applying spring brakes. Once the air tank on the trailer leaks down the trailer will roll freely.

 

[D6c]

All so look for mud dobber houses in the air lines.

Good point....I should remove the coil hoses and make sure they're open, as well as apply air compressor air to trailer lines to see if brakes operate.

 

[topo]

That's the same valve I have on both my trailers 1954 m269 and 1967 m131a5c. I think c-6 or r-6 relay valve .

 

[Scrounger]

It sounds like you have a trailer with the old system that uses air from the tank(s) to apply the brakes in the event of a breakaway. The air tank(s) even if completely empty should charge in less than five minutes if everything if working. There may be an obstruction i.e. mud dauber in the emergency airline on the trailer. I’ve had it happen before. I would disconnect the emergency line going into the trailer relay valve and check the air flow. Shop air comes in handy for this. The valve in the cab is a tractor protection valve and it will pop out if there isn’t enough air pressure in the whole system. That means air tank(s) on the trailer having enough air. If the airline is plugged, even if there is some air getting thru, it could take a very long time to charge the air tank(s) and the valve would pop out.
On a safety note. That trailer should have the wheels chocked when parking it or when coupling. The old way to couple to those trailers was to park in front of the trailer, hook up the airlines, charge the air, pull the trolley valve down, then back under the trailer.

 

[D6c]

Well, I checked the lines (not plugged)
*Checked the trailer brakes by hooking up to shop air (they worked properly)

* Put pressure gauge on service brake hose & emergency hose (got pressure to both)

* Checked trolley valve operation with gauge (got variable pressure with lever position)

What the heck?

Put it all back together and everything works now.

Haven't a clue what the problem was other than there might have been a sticky valve somewhere that finally let loose.
Nice that it fixed itself, but would like to have known what was wrong.

 

[Scrounger]

You didn’t post how long you held the tractor protection valve in. There may have been an obstruction that was cleared with the shop air. Flushing the system with something like CRC Brakleen probably would be a good idea. I had a mud dauber get in the emergency line of my XM971 and used Brakleen to clean the line. I was able to flush the line from both ends. It cleaned the line and the relay valve. One of the side benefits is now the air ride suspension doesn’t bleed down as fast when it is parked, after that I don’t hesitate to flush the line to remove any built up crud. These older trailers didn’t always have tractors with air dryers and it is amazing what can accumulate in the airlines, relay valves and what not.

 

[wreckerman893]

Murphy has a warped sense of humor.

 

[quickfarms]

Have you fist tried releasing the truck parking brakes then do the the trailer brakes.

 

[D6c]

You didn’t post how long you held the tractor protection valve in......

It may be that I wasn't holding it in long enough. Seems to take a minute or more to charge the brakes and until it's just about full you can't hear the air moving.
Even after the button stays in it takes a bit to get the trailer brake emergency circuit to release.

The good thing is the trailer air tank seems to hold pressure well. After shutting the truck down last evening it took several minutes for the service brakes to leak down enough for the button to pop out. Then it applies the emergency brakes on the trailer because the service brake pressure is low.
Went out this morning and the brakes were still holding....

Have you first tried releasing the truck parking brakes then do the trailer brakes?

I did try that but I'm not sure if it makes a difference or not. I was doing different things and didn't take note of where the part brake was set when I finally got things to work. I'll have to recheck that once I get my fuel lines sorted out (separate thread on fuel issues)

I'm getting close to getting all the small things fixed on the truck....anxious to give it a test drive in a few days. Won't go far as I don't have a registration or a CDL....maybe just to town to get some tire work done on the trailer.

 

[rosco]

The early trailers used a "relay emergency" trailer valve. When the trailer air supply drops below 60 psi, it will apply the trailers brakes, or, when you disconnect it from the supply, it will automatically apply the the trailer's brakes/as in a break-away. If you have the tractor's/truck's lines to the trailer crossed, the trailer's brakes will be applied.

One thing about a CDL, is that it give the recipient a working understanding of basic air brake systems. That is always reassuring when one is on the highway.

 

[D6c]

The early trailers used a "relay emergency" trailer valve. When the trailer air supply drops below 60 psi, it will apply the trailers brakes, or, when you disconnect it from the supply, it will automatically apply the the trailer's brakes/as in a break-away. If you have the tractor's/truck's lines to the trailer crossed, the trailer's brakes will be applied.

One thing about a CDL, is that it give the recipient a working understanding of basic air brake systems. That is always reassuring when one is on the highway.

I think I have a basic understanding of air brakes....just has taken a bit to sort out getting military truck working with civilian trailer and making sure everything is working.
As far as a CDL goes I haven't gotten one because I didn't have a truck. Now that I have a truck I have a reason to get one.....