M35A3 Air Brakes

[Navo]

Another day another question

So the more I drive my new M35A3 the more I would Like to know. So today I was looking over the air brake system and noted a few items.

First I noticed that I would turn the truck off at 120PSI and I would still hear air leaking near the dryer (at least I think thats where its coming from). Then after a few seconds it makes a sharp suction sound (valve closing) but I can still hear air. Thoughts?

Second item- Im assuming that in a perfect truck if you shut the truck off at 120PSI that it should remain there. I can shut the truck off and then a little over a day when I go back out and start up the buzzer is going off.

It seems to leak till around 90PSI then a slower leak starts till it gets to 0PSI.

Im attaching a video maybe you guys can chime in some more info!

Thanks!!

Air Brakes - YouTube

 

[emmado22]

Most trucks leak, but that doesnt make it OK. Take soapy water and start checking the fittings. Thats the only way your going to find them.

 

[Navo]

Most trucks leak, but that doesnt make it OK. Take soapy water and start checking the fittings. Thats the only way your going to find them.

Yea, I'm gonna be doing the spray bottle thing. But I didn't know if that air dryer was working like it was suppose to.

 

[MyothersanM1]

I know on my 5-Ton (M809 series) the governor will do something similar to what you get upon shut down. I am slowly but surely fixing slow leaks in my system, but I have never experienced a drop in pressure like you get while the engine is running. Seems like your air dryer is blowing down way to long. Watching your needle drop is kinda' scary. Maybe your air dryer is bad order. I don't know, just a guess.

 

[Navo]

I know on my 5-Ton (M809 series) the governor will do something similar to what you get upon shut down. I am slowly but surely fixing slow leaks in my system, but I have never experienced a drop in pressure like you get while the engine is running. Seems like your air dryer is blowing down way to long. Watching your needle drop is kinda' scary. Maybe your air dryer is bad order. I don't know, just a guess.

It's weird cause when Im driving it stays between 105PSI-125PSI?

 

[steelandcanvas]

Be sure to understand they are not air brakes, but air over hydraulic brakes. Navo have you downloaded the proper TM's for your truck yet? They could answer some future questions you may have, since the truck is new to you.

 

[Navo]

Be sure to understand they are not air brakes, but air over hydraulic brakes. Navo have you downloaded the proper TM's for your truck yet? They could answer some future questions you may have, since the truck is new to you.

Yes sir I have. Very useful stuff! However, I like asking the questions here because obviously there is more feedback from a community and you all can most likely identify if it is a problem or not.

I do understand that they are air over hydraulic, and to use the soap and water in a spray bottle to look for leaks etc. I just thought that taking a video and showing what I observed would help people understand what I was trying to explain.

I did use the search function on the site and didnt find any topics with what I was observing hence why I started this topic. If I missed it, I do apoligize.

 

[MyothersanM1]

Be sure to understand they are not air brakes, but air over hydraulic brakes. Navo have you downloaded the proper TM's for your truck yet? They could answer some future questions you may have, since the truck is new to you.

Agreed.

It's weird cause when Im driving it stays between 105PSI-125PSI?

Sounds like the governor is doing its job keeping the air pumped up. Like said, definitely check out the air dryer. Oh, and make it a habit of draining your air tanks after ops.

 

[Bigjer]

Hard to tell from the post, but sounds similar to what my truck was doing. Sounded like it was blasting off. It was the priority air valve was shot. Took it out of the system and it has been great since.

 

[kcollum]

It looks to me that the air dryer purge valve is not closing all the way. In your video at the 25 second mark you can see the air dryer purge the moist air and then you can hear the air leak. At 120 psi the air governor kicks off, there is a line from the governor to the dryer that opens the purge valve. A quick test would be to bypass the dryer by hooking up the air in line to the air out and see if the problem goes away. If that is the case than all that may be needed is to clean the junk from the valve or a rebuild kit for the dryer. Erick's has the rebuild kit.

Hope this helps.

 

[gimpyrobb]

Yup, air dryer rebuild time!

 

[Rustygears]

When the Compressor reaches 120psi and the comp governor located on the sde of the comp unloads the compressor, it also signals the dryer to purge. That is the explosive whoosh sound you hear behind the spare. All the time the compressor is not filling the main tanks, the air charge stored in the dryer vents overboard to remove the moisture that it collected while filling the truck air system. It will make a low gush type sound for up to a minute after the pop at compressor unload time. All of this is very normal and essential to proper dryer operation. When the comp governor senses system pressure at around 90 psi, you'll hear another pop and a sound as the dryer seals up and the lines recharge as the compressor comes on line and then starts to fill the tank.

At idle, all of these sounds are pretty noticeable and can raise attention. They are normal. High pitched hisses, like from leaking air, is not normal and is indicative of a leak. The A3 has a mix of copper and plastic air line and the main line from compressor outlet to dryer inlet is a combination of both copper and plastic, the copper being the front half and the a coupling and transition to plastic. The compression fittings for that transition often works loose over the years, so check it. There are many other fittings to also check and tighten.

A healthy A3 should promptly fill from 0 psi to 120 psi in under 60 seconds at idle from engine start. Once filled to 120 psi (CTIS off) and idling without any brake application, etc. the compressor should not cycle again for like at least a half hour or more. The only dynamic load on the air system (again ctis off) other than brakes is the steering assist and the radiator fan clutch. If the steer assist isn't leaking like a sieve, as 90% of surplus A3s do, there should be virtually no air use while idling at the curb.

If you have the compressor cycling frequently at curb idle, like every few minutes, you have a major leak and you will definitely gum up the air dryer with lots of gooey snot, that will then mess up your tanks and CTIS.

Search for my ther posts on A3 air leaks, steer assist air leak fixes, dryer servicing and CTIS operation.

A healthy A3 never has anything come out of the tank drain valves, even the wet side drain. The dryer, when happy, takes care of that issue before the wet tank.

 

[Rustygears]

When the Compressor reaches 120psi and the comp governor located on the sde of the comp unloads the compressor, it also signals the dryer to purge. That is the explosive whoosh sound you hear behind the spare. All the time the compressor is not filling the main tanks, the air charge stored in the dryer vents overboard to remove the moisture that it collected while filling the truck air system. It will make a low gush type sound for up to a minute after the pop at compressor unload time. All of this is very normal and essential to proper dryer operation. When the comp governor senses system pressure at around 90 psi, you'll hear another pop and a sound as the dryer seals up and the lines recharge as the compressor comes on line and then starts to fill the tank.

At idle, all of these sounds are pretty noticeable and can raise attention. They are normal. High pitched hisses, like from leaking air, is not normal and s indicative of a leak. The A3 has a mix of copper and plastic air line and the main line from compressor outlet to dryer inlet is a combination of both copper and plastic, the copper being the front half and the a coupling and transition to plastic. The compression fittings for that transition often works loose over the years, so check it. There are many other fittings to also check and tighten.

A healthy A3 should promptly fill from 0 psi to 120 psi in under 60 seconds at idle from engine start. Once filled to 120 psi (CTIS off) and idling without any brake application, etc. the compressor should not cycle again for like at least a half hour or more. The only dynamic lad on the air system (again ctis off) other than brakes is the steering assist and the radiator fan clutch. If he steer assist isn't leaking like a sieve, as 90% of surplus A3s do, there shoul be virtually no air use while idling at the curb.

If you hav the compressor cycling frequently at curb idle, like every few minutes, you have a major leak and you will definitely gum up the air dryer with lots of gooey snot, that will then mess up your tanks and CTIS.

Search for my ther posts on A3 air leaks, steer assist air leak fixes, dryer servicing and CTIS operation.

A healthy A3 never has anything come out of the tank drain valves, even the wet side drain. The dryer, when happy, takes care of that issue before the wet tank.

 

[Navo]

When the Compressor reaches 120psi and the comp governor located on the sde of the comp unloads the compressor, it also signals the dryer to purge. That is the explosive whoosh sound you hear behind the spare. All the time the compressor is not filling the main tanks, the air charge stored in the dryer vents overboard to remove the moisture that it collected while filling the truck air system. It will make a low gush type sound for up to a minute after the pop at compressor unload time. All of this is very normal and essential to proper dryer operation. When the comp governor senses system pressure at around 90 psi, you'll hear another pop and a sound as the dryer seals up and the lines recharge as the compressor comes on line and then starts to fill the tank.

At idle, all of these sounds are pretty noticeable and can raise attention. They are normal. High pitched hisses, like from leaking air, is not normal and s indicative of a leak. The A3 has a mix of copper and plastic air line and the main line from compressor outlet to dryer inlet is a combination of both copper and plastic, the copper being the front half and the a coupling and transition to plastic. The compression fittings for that transition often works loose over the years, so check it. There are many other fittings to also check and tighten.

A healthy A3 should promptly fill from 0 psi to 120 psi in under 60 seconds at idle from engine start. Once filled to 120 psi (CTIS off) and idling without any brake application, etc. the compressor should not cycle again for like at least a half hour or more. The only dynamic lad on the air system (again ctis off) other than brakes is the steering assist and the radiator fan clutch. If he steer assist isn't leaking like a sieve, as 90% of surplus A3s do, there shoul be virtually no air use while idling at the curb.

If you hav the compressor cycling frequently at curb idle, like every few minutes, you have a major leak and you will definitely gum up the air dryer with lots of gooey snot, that will then mess up your tanks and CTIS.

Search for my ther posts on A3 air leaks, steer assist air leak fixes, dryer servicing and CTIS operation.

A healthy A3 never has anything come out of the tank drain valves, even the wet side drain. The dryer, when happy, takes care of that issue before the wet tank.

Thank you VERY much for this informational post! So, it sounds like my air dryer is just fine. I will check the valve on the steering and see if it is leaking. If not, then I will start tracing the lines. Im currenly on my working shift and wont be able to check any of this out for at least 30 days...

 

[glcaines]

Try shutting off the air assist steering by closing the valve on the firewall. This will eliminate any leaks in the steering system and will help narrow down the problem. I had a similar problem with my A3 and had to replace the governor. When my governor was faulty, the pressure relief valve on top of the drivers-side air tank would discharge repeatedly, while at the same time the pressure gauge would still be reading zero. I had already replaced the air dryer with an NOS air dryer because the old one was physically damaged. Now the air system works great.

 

[Rustygears]

Good point about isolating the air assist with the little valve on the driver's side firewall. I wasn't going to repeat all the air assist regulator discussion......

As I recall, the same valve also controls air supply to the fan clutch. I don't remember if it engages or disengages with the air off.

 

[glcaines]

As I recall, the same valve also controls air supply to the fan clutch. I don't remember if it engages or disengages with the air off.

I'm not sure if it controls the fan clutch or not either. He can look it up in the TM or trace the line.

 

[Rustygears]

The fan clutch and air assist branch from the same line and I believe the same little needle valve at the firewall controls the supply for both. What I don't remember is whether the fan is permanently enabled with air supply off or disabled with air supply off.