Air leak under dash

[RaggedyMan]

I have been searching for the last hour and I know it's been asked a dozen times but it seems there has been no "final" answer (or more likely I missed it). All I found in the TM's were exploded views but nothing about how all this is supposed to work.

First I guess I should say this is a 65 M35A2C. There is an air manifold under the dash that connects electrically to the low pressure light/buzzer and air lines to/from whatever else. On the right side is a small hole that air constantly escapes. I read during my search that some say to plug it but when I press my finger over it the low pressure buzzer sounds and the light lights even though the pressure is between 90 & 120. This air loss doesn't affect the gauge pressure or interfere with any air operated items. The air continues to come out after I shut it down for a while and when it stops (or maybe I just can't hear it) there is still some pressure in the tanks. I can live with the hiss but I understand it can't be good on the compressor. I disassembled the govenor (engine side of the firewall) and cleaned it but nothing.

Does this "manifold" have working parts inside? Is this hole an evacuation for over pressure? Do I need to adjust the govenor even if the pressures are within limits? Is there something in/on the compressor I need to check?

 

[rustystud]

There should be no leaks at all from under the dash. Give us a picture of where this "hole" is OK ? Then we can better help you.

 

[RaggedyMan]

This is the offending part.
It is just right of the steering column. The offending hole is high to the right. The top wire runs over the steering column to I don't know where. The bottom line runs to the low pressure warning light.........



And the large supply line runs to.........

This manifold.

 

[Crestonean]

Mine also had a pretty good leak from that same hole when I bought it. I removed the sending unit and took it apart carefully. After cleaning all the old dust out, I reassembled it and it has been silent and trouble free for the past year.
Hope this helps.

 

[Crestonean]

Here's a photo of the unit taken apart -

 

[RaggedyMan]

Thank you Crestonean!
I pulled it off cleaned it up and....... it still leaks a little so I may just try to find one. Does anybody know what it's called, how to look it up? While looking over different suppliers the last week I've been keeping an eye open for them but never saw one, if I knew what I was looking for I might could find it. Gonna try again now so we'll see.

 

[gimpyrobb]

If it is leaking less, I'd try thread sealant.

 

[Crestonean]

If you can't get it to stop leaking, there's one listed on the Memphis Supply pages for the M35's. It's expensive though.

 

[RaggedyMan]

Thanks again Crestonian, just ordered one for around $27 plus shipping.

 

[frank8003]

The thing has to be here someplace.
If you get the NSN and or part number please post it.


TM2320-209-20P illustration 67
NSN 5930-00-058-1576
PN MS 27152-1

 

[RaggedyMan]

The thing has to be here someplace.
If you get the NSN and or part number please post it.
View attachment 575490

It is JJ on the diagram, part number MS27152-1

​

 

[RaggedyMan]

I installed a new switch yesterday, it even leaks at around 90psi but for the first time I still had 40-50 psi after sitting overnight. Today I adjusted the regulator to halfway between 90 and 120psi (it's always been near 120). Watching the gauge now I can hear the regulator cut in around 85ish and out around 110ish, then it drifts down and then back up.

So, if it's not suppose to leak at all why in the world is it? Seems my regulator is regulating fine but I still hear that dang leak? Is there something else I'm missing?

 

[Big Dumb Truck]

I had a few leaks under the dash. I replaced the old hoses with copper tube with flair fitting. The system would hold air for more than a day.

 

[rustystud]

I installed a new switch yesterday, it even leaks at around 90psi but for the first time I still had 40-50 psi after sitting overnight. Today I adjusted the regulator to halfway between 90 and 120psi (it's always been near 120). Watching the gauge now I can hear the regulator cut in around 85ish and out around 110ish, then it drifts down and then back up.

So, if it's not suppose to leak at all why in the world is it? Seems my regulator is regulating fine but I still hear that dang leak? Is there something else I'm missing?

You might have, as "Big Dumb Truck" did a leaking air hose somewhere. Take a spray bottle of soapy water ( or Windex ) and start looking for leaks.

 

[TGP (IL)]

Not to change the op thread but what is the line under the dash clipped to the firewall and dead ends over on the passenger side with
A valve and cap?
Looks like a 3/8 line. I can not find it in my TM's
Thanks
Tom

 

[RaggedyMan]

You might have, as "Big Dumb Truck" did a leaking air hose somewhere. Take a spray bottle of soapy water ( or Windex ) and start looking for leaks.

Thanks Rustystud, I did that the same day and did find a few leaks, most were loose connections. I'll be the first to tell you I'm new to how all this pressure stuff works but learning, still at a loss about the leaky switch (I keep calling it a "leaky" switch but the small open port would lead a newbie to believe it's suppose to allow air to escape) and am a bit confused about the correlation between the air coming out of that and what that has to do with other leaks.

Not to change the op thread but what is the line under the dash clipped to the firewall and dead ends over on the passenger side with
A valve and cap?
Looks like a 3/8 line. I can not find it in my TM's
Thanks
Tom

Good call TGP, I've been wondering the same...

 

[Wildchild467]

Check your hoses for cracks too. I think I had one of mine leak a long time ago. I have since replaced it and never had a problem since.

 

[steelandcanvas]

Not to change the op thread but what is the line under the dash clipped to the firewall and dead ends over on the passenger side with
A valve and cap?
Looks like a 3/8 line. I can not find it in my TM's
Thanks
Tom

It's the attachment point for the on-board tire inflation hose. It's better to use the Emergency gladhand at the back of the truck.

 

[TGP (IL)]

It's the attachment point for the on-board tire inflation hose. It's better to use the Emergency gladhand at the back of the truck.

Thanks!!
Was thinking that was what is was for, and your right about using the glad hand.
That's all I have ever used.
Tom

 

[frank8003]

The thing has to be here someplace.
If you get the NSN and or part number please post it.
View attachment 575490

TM2320-209-20P illustration 67
NSN 5930-00-058-1576
PN MS 27152-1

The way this works is you go get almost any
kind of compressor and hook it up to the starboard rear glad hand however you want to do it.
Don't have to run engine, just pump up the air by remote so to speak.
Operate the brakes, it likes it , play with all the air stuff, Snoop the leaks {stuff is $4.89 at HD}, fix everything and be on your way.

You need to know the brake system 100% of time......................

Oh, in that "fix everything might take a week" is shorthand for the week but then you be done.
a picture for the General