M939 No air pressure build up

Have seen several posts on this subject on not building air, I have a M939 that i removed the air discharge line from compressor (brass line) and could feel air pushing out of compressor (rules out bad compressor), installed new air governor and truck will not build air at all. Possible that the air dryer filter is bad?

Grab the line coming off the drivers side frame rail into the dryer. is it vibrating or clicking? means its building air to there if it is. Then just pop the drain ports on the side to see if you have air there? If you got the click into the dryer and no air on the drains then you know. The dryer if you have not changed it before could be stopped up. Just take it apart and clean it. It's not to difficult. The rebuild kits are available from Napa and Truck Pro pretty readily.

Is it a tractor? 939 covers a BUNCH of trucks. I recovered a tractor once that had the valve for the trailer opened. Talk about a "face palm".

This is a cargo truck, and i am pretty sure the air dryer has not been serviced, i did crack the air bleeds on passenger side, very little air came out, this was just released from service and is now owned by a museum. I found a rebuild kit for under 150.00 and will try that

Is air leaking out the bottom of the air dryer? If so just push up on the center of that is in side and it should stop.

Take the air line going into the dryer off see if air pressure. If so take the air line off the air dryer out see if pressure.

Thomas_G said:

This is a cargo truck, and i am pretty sure the air dryer has not been serviced, i did crack the air bleeds on passenger side, very little air came out, this was just released from service and is now owned by a museum. I found a rebuild kit for under 150.00 and will try that

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If you are considering a rebuild kit, might as well just purchase a brand new one. Member Suprman has some that fit just fine and depending on shipping costs, might be cheaper than the $150 price of the rebuild kit. I have one on my cargo truck now and it works great.

Primary, secondary, or both that aren't building air?

Thomas_G said:

Have seen several posts on this subject on not building air, I have a M939 that i removed the air discharge line from compressor (brass line) and could feel air pushing out of compressor (rules out bad compressor), installed new air governor and truck will not build air at all. Possible that the air dryer filter is bad?

Click to expand...

Make sure your glad hand valves are closed front and back.

You say you "could feel air pushing out of compressor"... I'm not sure this rules out the compressor, unless you actually put a gauge on it? Moving air doesn't necessarily = pressure.
Be careful if you check this without a tank or relief valve though. Maybe just crank the motor over while someone else watches the gauge...

Thomas_G said:

Have seen several posts on this subject on not building air, I have a M939 that i removed the air discharge line from compressor (brass line) and could feel air pushing out of compressor (rules out bad compressor), installed new air governor and truck will not build air at all. Possible that the air dryer filter is bad?

Click to expand...

Ok has the truck ever had pressure? Have you closed all 3 glad hands? Most likely culprit is valve on bottom of air drier, won't hurt to remove it and clean it, then put it back in.

sent from my decrepit fingers

Jbulach said:

Make sure your glad hand valves are closed front and back.

You say you "could feel air pushing out of compressor"... I'm not sure this rules out the compressor, unless you actually put a gauge on it? Moving air doesn't necessarily = pressure.
Be careful if you check this without a tank or relief valve though. Maybe just crank the motor over while someone else watches the gauge...

Click to expand...

You can put shop air to the front glad hand to see if the system will hold air. You might even be able to hear where its leaking if not!

I removed the purge valve and cleaned up the bore, it was pretty tight with very little debris, I resealed and going to put on this weekend, hopefully that may of been the problem. I was told it would air up before, the desiccate filter was like new.

Removed lower section and cleaned, resealed Purge valve and truck was able to build air.

Thanks for the update! We always like to hear what fixed it.

Thomas_G said:

Removed lower section and cleaned, resealed Purge valve and truck was able to build air.

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If you want to have a purge valve kit I have some I am willing to sell.