brake light oddity

[Lmtv772]

No pressure in the lines = no brake light
pressure in the lines means = constant brake light
I replaced all the relays
and the brake light switches
and the indicator stalk
and the light switch
yet, as soon as there is air in the system the brake lights come on.
Do I need to replace the brake pedal valve assembly?

 

[Suprman]

Can you put the light switch in blackout mode and do the same brake light check.

 

[Lmtv772]

Yes, blackout brake light also comes on in blackout mode when air pressure is up. I replaced the both brake light pressure switches

 

[Suprman]

The manual has everything listed for something non working but nothing for working too much. It does list the 2 diode arrays in the power panel left side as a possible culprit for lights not working. I wonder if one could short and cause odd symptons. Couldnt hurt to switch them and see what happens.

 

[Lmtv772]

The manual has everything listed for something non working but nothing for working too much. It does list the 2 diode arrays in the power panel left side as a possible culprit for lights not working. I wonder if one could short and cause odd symptons. Couldnt hurt to switch them and see what happens.

Roger that, I have read through the TM as well and couldn't find anything about non stop working. I did swap the diodes, as well. Then I posted about my brake light issues today.
If there were a short, the lights would not come on, or would stay on even if the air is purged.
So I know It can't be a short.

 

[Suprman]

I have had to replace a treadle valve before. It was spongy but the brakes did function. With the replacement valve normal brake pedal feel returned. Maybe air is seeping somewhere inside. If you disconnect the electrical wires from the 2 switches mounted on the valves, then ohm meter them with no air in the system and also with air then you could do the same on another truck that works and compare. I metered some extras I have here they appear to be normally open with no air.

 

[Lmtv772]

I have had to replace a treadle valve before. It was spongy but the brakes did function. With the replacement valve normal brake pedal feel returned. Maybe air is seeping somewhere inside. If you disconnect the electrical wires from the 2 switches mounted on the valves, then ohm meter them with no air in the system and also with air then you could do the same on another truck that works and compare. I metered some extras I have here they appear to be normally open with no air.

Maybe I'll take the one out of the M1088, They should be the similar (same), right?

 

[Suprman]

Yes I swapped one from a 1088 to a 1083 it was identical. I did the whole assembly with pedal it was easier that way.

 

[Lmtv772]

Yes I swapped one from a 1088 to a 1083 it was identical. I did the whole assembly with pedal it was easier that way.

Hooah
how long did it take you to do the swap?
Now that you mention it, the M1088 we have has a much firmer brake pedal. It's like stepping on a brick rather than a cardboard box.

 

[Suprman]

It took about 2 hours between taking one out of the parts truck and then swapping it into the 1083. If you can do the whole assembly its alot easier. If the treadle valve is spongy at all its bad. It should be nice and firm even with no air in the system. I went back and forth between trucks at first when it didnt feel right to me just to make sure I wasnt imagining things.

 

[Lmtv772]

It took about 2 hours between taking one out of the parts truck and then swapping it into the 1083. If you can do the whole assembly its alot easier. If the treadle valve is spongy at all its bad. It should be nice and firm even with no air in the system. I went back and forth between trucks at first when it didnt feel right to me just to make sure I wasnt imagining things.

on all the LMTV it feels kind of spongy, but all the LMTV have valve issues, minus mine. The 2004 M1088 doesn't have that, the pedal is firm and has a definite end, when you step on it. And the brakes apply in a very sensitive manner. On my LMTV I have to step into it a bit more.
I think I'll swap it out tomorrow after work and see what that does.

 

[Lmtv772]

So Suprman
I came to realize that the brake circuit on my brake valve assembly was switched (as per TM and all our LMTV), but that's beside the point.
The M1088 brake valve looks a lot bigger and seem to mount vertical instead of horizontally to the support ( which can work if I modify it slightly) < Just wanted to make sure it's the same part you've used.

 

[Suprman]

Your 1088 is an A1 and the setup looks different. The brake valve itself should operate the same though. Here is a pic, on the left is the bad valve I removed from the 1083, the 1088 valve was identical. On the right is a spare I have from a 2007 M1083A1.

 

[Lmtv772]

I think my first order of business will be to swap the lines to the way they were supposed to be connected.
The I have a good hard thinking about the larger set up of the M1088. You're right , the part came from a 2004 M1088A1

 

[Suprman]

I know where to find a new valve piece itself. The cost isnt too bad. You can email me will atpawnking dotcom I cant post it in the forum
Will

 

[Lmtv772]

I know where to find a new valve piece itself. The cost isnt too bad. You can email me will atpawnking dotcom I cant post it in the forum
Will

trying to figure out first if that would fix my brake light.I'll keep your suggestion in mind.
Thank you, Will
~michael

 

[Suprman]

Tomorrow I can take apart my bad valve and see whats going on inside there. Maybe give some insight into how air moves thru it.
Will

 

[Lmtv772]

I think there maybe an internal leak or so. as soon as I put pressure into the system the lights come on, drain the air and the lights go off.

 

[Lmtv772]

So I went through the scary part and swapped the brake valve to the M1088 one (which I had to modify a bit) I also swapped out the fittings from the A1. I like those a lot better- you just have to push in the hoses and they stay and seal.
The good news is the brake lights seem to work as expected again.

 

[Overdrive]

That is great you figured it out! I am going to add the issue to my list of warning signs something is going bad.