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Valve behind heater box?

Oski1042

Member
42
47
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Location
Moscow ID
So with the differential relay solenoid question out of the way, it was time for another fun LMTV truck issue. Recently I could hear air escaping from the area just forward of the fresh air intake. I took the grill off and found the culprit. There is a valve that extends upward. Behind the heater box I can see an air line that goes into the valdex valve and one that goes out and connects to the large valve above the brake pedal. When I shut off the engine I can hear a steady stream. I guess I am wondering if anyone recognizes this problem? If so .... should I endeavor to rebuild it ... or just replace it? As always ... thank you!
 

Oski1042

Member
42
47
18
Location
Moscow ID
Yup ... it was just air leaking out of the back of the brake pedal valve.

Now I know where that valve goes to vent! Pretty sure it is suppose to be just for purging ... not slow leaks emptying your tanks!

20220220_153642_resized.jpg
 

Ronmar

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Port angeles wa
Correct, that valve should only vent air when you release the pedal. Take the treadle valve apart and clean it. If it still leaks, install a parts/rebuild kit.

The vent line actually attaches to a T before that port thru the floor. The park and trailer control valves also vent out thru that same port. Disconnecting that line at the treadle valve is the best way to decide which of the 3 valves is actually leaking…
 

19Detail

Member
77
65
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Location
Vermont
Yup ... it was just air leaking out of the back of the brake pedal valve.

Now I know where that valve goes to vent! Pretty sure it is suppose to be just for purging ... not slow leaks emptying your tanks!

View attachment 859578
Thanks for the info. Just to check my understanding, are you saying the hose in your hand is the other side of the vent that is just above the front grill in the center of the truck? I had a surprise leak in mine that went away after about 10 min. (it would not leak if I pressed on the brake pedal). I am assuming it stopped because the truck warmed up ( I am in a cold climate). I was dreading trying to get to where I heard the air escaping because it is in a tight spot.
 

Ronmar

Well-known member
2,515
4,800
113
Location
Port angeles wa
Thanks for the info. Just to check my understanding, are you saying the hose in your hand is the other side of the vent that is just above the front grill in the center of the truck? I had a surprise leak in mine that went away after about 10 min. (it would not leak if I pressed on the brake pedal). I am assuming it stopped because the truck warmed up ( I am in a cold climate). I was dreading trying to get to where I heard the air escaping because it is in a tight spot.
That is correct. It should only have air coming out that port when you release the pedal or release the park air to set the park brakes, and then only as long as it takes to vent all the braking air. There should be no steady leak. If there is, it is either leaking past the treaddle valve or past the park/trailer valve which also vents out that port above the radiator.

The treadle valve seals sometimes get crap in them, or the seals deterioriate, or the cold could have caused them to be stiff. The Orings up in the park and trailer valve wear out/dry out and deteriorate… Pulling the hose he is holding in the pic is a good way to determine which valve group is leaking out that front port…
 

wandering neurons

Active member
238
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Location
Fallon, NV
I had that specific problem on my M1081: colder weather I'd have venting from behind the grille. There's another thread that describes how to test which chamber is leaking if it is truly the treadle leaking and not a park/trailer valve. The back-most section of the treadle is serviceable in the truck, but not the other half, the treadle must be removed. Post #57 in this thread (https://www.steelsoldiers.com/threads/lmtv-air-brake-problems.154348/page-3) has some diagnostics.
I fixed my truck by ordering the following: "Brake Pedal Valve Seal Kit For LMTV / MTV" from one of the suppliers here in SS. If Google/DuckDuckGo is of no help, PM me and I'll reply with the supplier's name.
The PMs tell how to remove the treadle valve and foot pedal - it really needs two people as you have to displace the dash assembly and reach into the bottom of the dash box, in addition to removing fasteners from the underside at the same time. Need three hands or looooong arms.
There's no documentation as to disassemble/reassemble of the treadle valve itself, but it's really simple if you go slowly. Mostly snap-rings and springs holding the thing together. I had two pieces in the kit that were not replaceable, FYI. And I used air tool lube for the rings and valves during reassembly.
 
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