Air leak/bleed down

[Goliath P. Jones]

Well it was bound to happen, lol. Brought home my m939 on Tuesday, and Wednesday I noticed when I fired it up that the low air light and buzzer was going off. Looked at the air pressure gauges and they're zero psi. The compressor works, got the psi up to 90. I could here a hiss from behind the dash, called up the guy that I bought it from and asked him if he had any ideas. He said if it's from behind the dash it would likely be the switches for the wipers and that the switches are very temperamental and to adjust them until the hiss goes away. The night before when I bought it, he was showing me how they operated. So it sounded like an easy fix, I got the switches in their sweet spot, no hissing anymore. Well the next morning I go out and check on it. Air gauges back to zero, fire up the truck, pressure goes up to 120 psi, both primary and secondary. A couple hours later the psi is noticeably lower, and I'm sure by tomorrow morning it will be back to zero. The seller said that its never done this before, and I've only driven it 45 miles, not to say that something couldn't have had a fitting come loose, a small pin hole occurred or something like that. So I think it's safe to say that it should hold psi indefinitely? In talking with my dad that drove trucks like this in the military air leaks are common. I have crawled all over, under and around and dont hear any other hissing, I'm thinking about taking a squirt bottle of water with a few drops of dish soap mixed in to spray on the fittings and rubber hoses to search for the leak via any bubbles forming. Any additional advice someone with more experience can give me to help locate or pinpoint the leak? Thanks for any help, suggestions and advice in advance.

 

[Valor]

Well it was bound to happen, lol. Brought home my m939 on Tuesday, and Wednesday I noticed when I fired it up that the low air light and buzzer was going off. Looked at the air pressure gauges and they're zero psi. The compressor works, got the psi up to 90. I could here a hiss from behind the dash, called up the guy that I bought it from and asked him if he had any ideas. He said if it's from behind the dash it would likely be the switches for the wipers and that the switches are very temperamental and to adjust them until the hiss goes away. The night before when I bought it, he was showing me how they operated. So it sounded like an easy fix, I got the switches in their sweet spot, no hissing anymore. Well the next morning I go out and check on it. Air gauges back to zero, fire up the truck, pressure goes up to 120 psi, both primary and secondary. A couple hours later the psi is noticeably lower, and I'm sure by tomorrow morning it will be back to zero. The seller said that its never done this before, and I've only driven it 45 miles, not to say that something couldn't have had a fitting come loose, a small pin hole occurred or something like that. So I think it's safe to say that it should hold psi indefinitely? In talking with my dad that drove trucks like this in the military air leaks are common. I have crawled all over, under and around and dont hear any other hissing, I'm thinking about taking a squirt bottle of water with a few drops of dish soap mixed in to spray on the fittings and rubber hoses to search for the leak via any bubbles forming. Any additional advice someone with more experience can give me to help locate or pinpoint the leak? Thanks for any help, suggestions and advice in advance.

My 923A2 truck leaks down the air too over night. There are so many air connections that it's no wonder it doesn't leak more! I havn't taken a squirt bottle with dishsoap and water and sprayed around but maybe I should. Check your air drain valves on the passenger side behind and below the passenger door for tightness. You can check the TM's for air leak tests too. I don't remember where they are but I did see them. As long as you don't have a major air leak on your truck, I think you will be okay. Nice truck you have there!

 

[jasonjc]

In theory yes it should hold air indefinitely. In the real world no. If it leaks down over night no big deal. But if it leaks down in minutes or hour then I'd worry about it.

 

[Mullaney]

Well it was bound to happen, lol. Brought home my m939 on Tuesday, and Wednesday I noticed when I fired it up that the low air light and buzzer was going off. Looked at the air pressure gauges and they're zero psi. The compressor works, got the psi up to 90. I could here a hiss from behind the dash, called up the guy that I bought it from and asked him if he had any ideas. He said if it's from behind the dash it would likely be the switches for the wipers and that the switches are very temperamental and to adjust them until the hiss goes away. The night before when I bought it, he was showing me how they operated. So it sounded like an easy fix, I got the switches in their sweet spot, no hissing anymore. Well the next morning I go out and check on it. Air gauges back to zero, fire up the truck, pressure goes up to 120 psi, both primary and secondary. A couple hours later the psi is noticeably lower, and I'm sure by tomorrow morning it will be back to zero. The seller said that its never done this before, and I've only driven it 45 miles, not to say that something couldn't have had a fitting come loose, a small pin hole occurred or something like that. So I think it's safe to say that it should hold psi indefinitely? In talking with my dad that drove trucks like this in the military air leaks are common. I have crawled all over, under and around and dont hear any other hissing, I'm thinking about taking a squirt bottle of water with a few drops of dish soap mixed in to spray on the fittings and rubber hoses to search for the leak via any bubbles forming. Any additional advice someone with more experience can give me to help locate or pinpoint the leak? Thanks for any help, suggestions and advice in advance.

The really hard part of finding a leak on one of these trucks is all the noise that happens around you while it is running. Best plan - if you have a compressor - is to attach it to the driver's side (front) glad hand. Get a glad hand and a quick connect to make it easy on yourself. Hook that up and you might actually find you leak by sound.



My experience with my first air leak was the air tank under the door. Metal tank lays on a metal plate on my M936 and is held in place with two U-Bolts. Lots of years and accumulated dirt and maybe road salt ate the thinner metal (my air tank). With the truck OFF and my shop air compressor running about a hundred feet away - I went right to the problem.

The "IF" is having a compressor. Doesn't have to be big. Just something to pump up the air system.

I personally prefer to spend the effort to get the air system as "air tight" as it can be.
A spray bottle with soapy water is a good idea and works well.


Good luck on your hunt!

.

 

[Goliath P. Jones]

I went out and checked it last night a few hours after shutting it down and all the air had leaked out. The truck came with a glad hand connection for a compressor hose and I do have a couple compressors so I will try that approach first with a squeeze bottle of water and soap, as I live on a fairly busy road the traffic noise is nonstop even when I am hundreds of feet off the road. Lol, semi trucks and there air brakes slowing down to stop at the traffic lights or motorcycles trying to see how fast they can get going to pass cars. I will definitely keep it posted if and when I find the leak or leaks. Thanks for the advice.

 

[Mullaney]

I went out and checked it last night a few hours after shutting it down and all the air had leaked out. The truck came with a glad hand connection for a compressor hose and I do have a couple compressors so I will try that approach first with a squeeze bottle of water and soap, as I live on a fairly busy road the traffic noise is nonstop even when I am hundreds of feet off the road. Lol, semi trucks and there air brakes slowing down to stop at the traffic lights or motorcycles trying to see how fast they can get going to pass cars. I will definitely keep it posted if and when I find the leak or leaks. Thanks for the advice.

.
That is great!

With your truck off - and if you have a steady leak - with your compressor attached it should make your chances better.
Even with a noisy road nearby you might find the problem before you break out the soapy water.
More so if the leak is inside the cab...

 

[simp5782]

Standard is no more than 5lbs loss over 3mins.

Dash item leaks will drain the tanks to 60psi the ppv then closes air to that system so if your tanks hold 60 or more you know where to start.

I can educate you on everything else at a later date.

The truck has more air lines than you have arteries.

 

[Mullaney]

Standard is no more than 5lbs loss over 3mins.

Dash item leaks will drain the tanks to 60psi the ppv then closes air to that system so if your tanks hold 60 or more you know where to start.

I can educate you on everything else at a later date.

The truck has more air lines than you have arteries.

.
Thanks Wes!

I did some quick math. 120# divided by 5# (of loss in 3 minutes) = 24 units of "3 minutes of air loss"
That comes to 72 minutes to zero air with the truck off.

Is my math bad or is that just minimal functional levels of loss?
Assuming the leak is beyond the PPV (Pressure Protection Valve)?



Tim

 

[simp5782]

I don't have enough fingers and my shoes are still on

 

[Goliath P. Jones]

Standard is no more than 5lbs loss over 3mins.

Dash item leaks will drain the tanks to 60psi the ppv then closes air to that system so if your tanks hold 60 or more you know where to start.

I can educate you on everything else at a later date.

The truck has more air lines than you have arteries.

I was going to hook up the air compressor to it today, but its been nonstop driving rain today. But its definitely draining down completely, so I guess it is a safe bet that its not a dash item leaking. Though I did start it up, let it recharge the psi, and the one thing that I have noticed every time I shut it down is that the wiper switches are always out of sync from where I left them and they continue to allow air to bleed by. So I have to at least assume that it's part of the problem. As I sit in the truck, and sweet spot dial the dash switches until they stop bleeding by. But then as soon as the tanks build up the pressure again they start leaking.

 

[Goliath P. Jones]

Standard is no more than 5lbs loss over 3mins.

Dash item leaks will drain the tanks to 60psi the ppv then closes air to that system so if your tanks hold 60 or more you know where to start.

I can educate you on everything else at a later date.

The truck has more air lines than you have arteries.

Would starting with pressurizing the system via the glad hand still be the best place to start? I know little to nothing about the system beyond the air pressure running almost everything. But I am definitely interested in learning as much as I possibly can about it.

 

[fasttruck]

As suggested in post 7 an airbraked truck can loose all its air in a couple of hours and be legal Fire Departments and such interested in fast response times plumb a shoreline in the maintain air pressure when the vehicle is in quarters and shut off. The biggie is how fast air leaks off when the parking brake is released and service brake applied.

 

[Goliath P. Jones]

As suggested in post 7 an airbraked truck can loose all its air in a couple of hours and be legal Fire Departments and such interested in fast response times plumb a shoreline in the maintain air pressure when the vehicle is in quarters and shut off. The biggie is how fast air leaks off when the parking brake is released and service brake applied.

I drove it to the yard debris recycling center today with the bed stacked full of debris, it doesn't lose any air with the parking brake off and the service brake applied that I noticed. Making a few more runs in it today to the recycling center and will definitely keep an eye on it to be sure.

 

[fasttruck]

The applicable standard for a straight truck is 2 pounds per minute with an air operated parking brake released and 3 pounds per minute with service brake applied. Any audible air leak likely will exceed the standard. Add 1 pound/min for a trailer truck. How you are supposed to measure a 3 pound leak on a gauge graduated in 20 pound increments is another problem.

 

[Goliath P. Jones]

The applicable standard for a straight truck is 2 pounds per minute with an air operated parking brake released and 3 pounds per minute with service brake applied. Any audible air leak likely will exceed the standard. Add 1 pound/min for a trailer truck. How you are supposed to measure a 3 pound leak on a gauge graduated in 20 pound increments is another problem.

I definitely noticed that with the park brake off and the service brake applied it drops a little pressure, which I would assume is to be expected. But after about a 10 pound drop in pressure the compressor kicks on and runs the psi back up to the 120 psi mark. My son says he thinks he can hear an air seep at or around the tank area under the passenger step up. So I am going to start the search for it there. Just have to get a day when it's not pouring rain, as it just barely won't fit through the shop door.

 

[Mullaney]

I definitely noticed that with the park brake off and the service brake applied it drops a little pressure, which I would assume is to be expected. But after about a 10 pound drop in pressure the compressor kicks on and runs the psi back up to the 120 psi mark. My son says he thinks he can hear an air seep at or around the tank area under the passenger step up. So I am going to start the search for it there. Just have to get a day when it's not pouring rain, as it just barely won't fit through the shop door.

.
Could be as simple as a partially open drain on the tank under the step.

I had a leak under my driver's step on my M936. (see pictures) Really couldn't tell where it was coming from until it was aired up on the front gladhand. Truck off for less noise. My particular situation was metal tank on metal skid plate under the step + YEARS of crud collected between the two.



It is really hard to tell where the noise is coming from with the truck running.


Tim

 

[Goliath P. Jones]

.
Could be as simple as a partially open drain on the tank under the step.

I had a leak under my driver's step on my M936. (see pictures) Really couldn't tell where it was coming from until it was aired up on the front gladhand. Truck off for less noise. My particular situation was metal tank on metal skid plate under the step + YEARS of crud collected between the two.

View attachment 827681 View attachment 827682 View attachment 827683 View attachment 827684

It is really hard to tell where the noise is coming from with the truck running.


Tim

That's a good amount of contact between the tank and plate. If that's the case with mine I'll be in good shape as the truck came with an extra air tank in the pile of parts.

 

[fasttruck]

The CDL airbrake inspection from which the pressure standards come from is started with the air pressure up, the engine off and the battery switch on. It is a test to detect leakage so it should not be conducted in a noisy area. When the parking brake is initially released or the service brake initially applied the air pressure in the reservoir will drop as the brake lines and chambers fill and then it should stabilize. Unless there is a leak which will cause the pressure to continue to drop.

 

[Goliath P. Jones]

Finally got a warm, dry and quiet day to track down the leaks on the 5 ton air system. I say leaks, not leak because it had six leaks that I have found so far. Hopefully that's all of them. Three were the plastic line fittings at the tanks under the passenger step. Two were the fittings into the tank, one small brass line fitting. The other is the T valve type fitting into a tank. Got five of them airtight, the T valve one slowed down. But I will have to undo the lines going into it to get it completely cleaned up and tight. The soap and water squirt bottle was the champion of the process. Put air to the lines and start squirting down everything that is air line related, tank related and in between.

 

[Mullaney]

Finally got a warm, dry and quiet day to track down the leaks on the 5 ton air system. I say leaks, not leak because it had six leaks that I have found so far. Hopefully that's all of them. Three were the plastic line fittings at the tanks under the passenger step. Two were the fittings into the tank, one small brass line fitting. The other is the T valve type fitting into a tank. Got five of them airtight, the T valve one slowed down. But I will have to undo the lines going into it to get it completely cleaned up and tight. The soap and water squirt bottle was the champion of the process. Put air to the lines and start squirting down everything that is air line related, tank related and in between. View attachment 829635View attachment 829636View attachment 829637View attachment 829638View attachment 829639

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Yeah! There you go! Congratulations. Good deal. Sometimes it isn't scientific.
Just a lot of elbow grease - or in this case maybe Dawn, Water and Time.

Happy to hear that you found a lot of your leaks.

Be sure to spray around the bottom of your air tanks too. Just in case.