Front unloader valve.

[Dave5Cs]

Look like this maybe white or blue 2 large air lines one at the or near the top and the other near the bottom. Purge valve on the bottom middle with a little rubber disk on it. That can be unscrewed and a new purge valve replaced or clean and rebuild the old one. It will let all the air out and it won't build up with it opened. Wack it with a stick while it is airing up and sometimes they close.

http://www.trucksupplyofsc.com/wp-content/themes/trucksupplysc/images/tech-tips/TSE-Tech-Tips-AIR-DRYER-SERVICING.pdf

[eagle19952]

is it frozen ?
...try hitting it with a heat gun.

[Rayj1954]

Look like this maybe white or blue 2 large air lines one at the or near the top and the other near the bottom. Purge valve on the bottom middle with a little rubber disk on it. That can be unscrewed and a new purge valve replaced or clean and rebuild the old one. It will let all the air out and it won't build up with it opened. Wack it with a stick while it is airing up and sometimes they close.

http://www.trucksupplyofsc.com/wp-content/themes/trucksupplysc/images/tech-tips/TSE-Tech-Tips-AIR-DRYER-SERVICING.pdf

Thank you very much... looks like it.  Helps me a lot...  I'll try the stick.

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[gumpy]

That's the purge valve on the air dryer. More than likely your governor is stuck (though it could be the purge valve is stuck).

Suggest you either replace the governor (about $15 from a truck parts place) or maybe just try tapping the governor with a rubber mallet or the plastic handle of a big screw driver. It might free it up.

The governor is located on the air compressor which is located on the back of the engine above the transmission. Access through the floor or through the right hand engine access door.

[Rayj1954]

That's the purge valve on the air dryer. More than likely your governor is stuck (though it could be the purge valve is stuck).

Suggest you either replace the governor (about $15 from a truck parts place) or maybe just try tapping the governor with a rubber mallet or the plastic handle of a big screw driver. It might free it up.

The governor is located on the air compressor which is located on the back of the engine above the transmission. Access through the floor or through the right hand engine access door.

If the governor is bad or stuck could that cause this purge valve to stay open? I can hear the air coming out of this valve, I haven't whacked it yet but it is most definitely open.

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[Rayj1954]

Thank all of you. You've been most helpful, and it is most appreciated.

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[Geoff]

The valve that is leaking from the middle front has to be the front brake valve. Just swap it out.

[gumpy]

The valve that is leaking from the middle front has to be the front brake valve. Just swap it out.

Brake valve?  Not located in front of the axle. That's where the dryer is on an MC9.

[gumpy]

If the governor is bad or stuck could that cause this purge valve to stay open? I can hear the air coming out of this valve, I haven't whacked it yet but it is most definitely open.

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If it's stuck in the unload state, it can cause the dryer purge valve to remain open and constantly purging. There's a shuttle inside the governor which controls the purge cycle on the dryer when it moves to unload. Purge valve closes when shuttle moves back to compressing. When this happened to me, pressure would build to about 60 psi or so and then the purge valve would open and pressure would drop way down. I whacked to governor with the handle of a large screwdriver and it would free up the shuttle. This only happened when it was cold out. Once it started getting some warm air through the governor, it would stay free till it was shut off and got cold again. Eventually I rreplaced the governor and then rebuilt the one that was sticking. Reinstalled it and it's been fine for years.

[Geoff]

Pressure protection valve?

[gumpy]

Pressure protection valve?

No, he already indicated it's from the air dryer purge valve.

[buswarrior]

So???

What happened?

happy coaching!
buswarrior

[chessie4905]

Probably thawed out and started working😉