Air Fails To Build Intermittently; Parking Brake?

gottaluvit · Jul 15, 2015

Okay. Running engine at idle this go 'round and full pedal. Primary 128 to 118. Secondary 128 to 122. I did hear the engine (actually compressor) get quiet when dryer purged. It is actually doing pretty good now at cutting in. Odd. Just fanned it to make low air buzzer kick in and it cut in. Time for a test drive. Maybe the gov teardown did something.

Rmtaunton · Jul 15, 2015

Let us know , we are routing for you or as we say in GA ya

gottaluvit · Jul 15, 2015

Rmtaunton said:
Let us know , we are routing for you or as we say in GA ya

Thank you very much. Well, it was a no go. Fanned brakes one more time before pulling out and it wouldn't build pressure. Drained tanks with it running and they completely drained to zero. I'm over it for the day. Gonna get a new governor and see how that goes. Classified search now. More spare parts, I suppose.

Swamp Donkey · Jul 15, 2015

gottaluvit said:
To date: New MRAP air dryer, cleaned and lubed the unloader valve, new ppvalve, checked all glad hand valves, fixed a leaky line that has air pressure only with parking brake in released position, flushed the entire system with air brake anitfreeze, took governor apart to look for blockage/crud (looked pretty clean), banged head on bumper several times (didn't want to dent the sheet metal!).

Any suggestions appreciated.

Could there actually be some other valve of some type messing with me?

Jim

There are 3 one way check valves on your truck. Every tank has one except the wet tank. I would check those out assuming the things you listed above all checked out. One of them could be hanging closed under regular pressure drops. Then, when you drain the tanks, the pressure differential is larger which causes it to open. Once pressure is built up, the differential is less and it hangs closed, until you drained the tanks again. Just a theory of course...but there have been problems with them before.

This is what they look like. Part# KN-23000 is a pretty common number for them and should easily cross if you need to. They're a very simple valve, block off plate and spring, but they can stick.

One is on the driverside behind the step as seen in the picture. That is the air filter canister on the left in the photo for reference.

The other two are on the forward end of each tank on the passenger side behind the tool box. Both can be seen in this picture.

The other thing I would do is install an independent pressure gauge before the gauges, valves and tanks. Maybe the govenor is not able to see a pressure drop, which would cause the compressor not to cut in. Just another theory, but checking pressures in other parts of the system might help you pinpoint the problem.

The HUlk · Jul 15, 2015

When the governor senses high pressure(130psi), it directs air pressure to the unloader through a tube which pushes open the unloader valve. All of the air the compressor makes then escapes out the unloader outlet port.

When the pressure drops below the low pressure threshold(90psi), the governor stops feeding air into the unloader and the unloader valve closes by way of an internal spring. All of the air the compressor makes is then forced out the main output tube to the air dryer.

You can test the unloader valve by simply removing the governor tube and main compressed air output tube while it's running. In this state the comprssor air should be coming out the main output tube to the air dryer as if trying to pump up the system. If there is no air coming out of the main compressor output tube, then the unloader valve is stuck in the unload state.

This happened to me because there was sticky goo on the sliding valve mating surfaces and the spring was not able to push the valve closed...sometimes. And it seemed to work better cold and woud start sticking when warmed up. I attribute that to the sticky goo getting stickier at higher temperatures. The goo was not apparent when removing the unloader top. There is a seperate metal piece that slides onto the unloader top and the goo was between these pieces. They should slide freely and I had to pull hard to seperate them. Notice black tar looking stuff in the photo.

Ford Mechanic · Jul 15, 2015

The HUlk said:
When the governor senses high pressure(130psi), it directs air pressure to the unloader through a tube which pushes open the unloader valve. All of the air the compressor makes then escapes out the unloader outlet port.

When the pressure drops below the low pressure threshold(90psi), the governor stops feeding air into the unloader and the unloader valve closes by way of an internal spring. All of the air the compressor makes is then forced out the main output tube to the air dryer.

You can test the unloader valve by simply removing the governor tube and main compressed air output tube while it's running. In this state the comprssor air should be coming out the main output tube to the air dryer as if trying to pump up the system. If there is no air coming out of the main compressor output tube, then the unloader valve is stuck in the unload state.

This happened to me because there was sticky goo on the sliding valve mating surfaces and the spring was not able to push the valve closed...sometimes. And it seemed to work better cold and woud start sticking when warmed up. I attribute that to the sticky goo getting stickier at higher temperatures. The goo was not apparent when removing the unloader top. There is a seperate metal piece that slides onto the unloader top and the goo was between these pieces. They should slide freely and I had to pull hard to seperate them. Notice black tar look stuff in the photo.

Very good info!

gottaluvit · Jul 15, 2015

The HUlk said:
When the governor senses high pressure(130psi), it directs air pressure to the unloader through a tube which pushes open the unloader valve. All of the air the compressor makes then escapes out the unloader outlet port.

When the pressure drops below the low pressure threshold(90psi), the governor stops feeding air into the unloader and the unloader valve closes by way of an internal spring. All of the air the compressor makes is then forced out the main output tube to the air dryer.

You can test the unloader valve by simply removing the governor tube and main compressed air output tube while it's running. In this state the comprssor air should be coming out the main output tube to the air dryer as if trying to pump up the system. If there is no air coming out of the main compressor output tube, then the unloader valve is stuck in the unload state.

This happened to me because there was sticky goo on the sliding valve mating surfaces and the spring was not able to push the valve closed...sometimes. And it seemed to work better cold and woud start sticking when warmed up. I attribute that to the sticky goo getting stickier at higher temperatures. The goo was not apparent when removing the unloader top. There is a seperate metal piece that slides onto the unloader top and the goo was between these pieces. They should slide freely and I had to pull hard to seperate them. Notice black tar looking stuff in the photo.

Thank you. I had already been through that drama. That was a beast to get back together. Mine was stuck open and no air pressure at all. I am hoping that is not the case again but heck, I only put about 30 miles on it since that task. Mine had that same goo. It was sticky too. I did take the steel line off the top and shoot a shot of aerokryl (no doubt mispelled and always mispronounced) in an orange can down there just this evening after giving up for the day. I also bought a new bendix brand governor so that should be in next week. Just eliminating and knowing what is new and what is still the big question mark!

gottaluvit · Jul 15, 2015

Swamp Donkey said:
There are 3 one way check valves on your truck. Every tank has one except the wet tank. I would check those out assuming the things you listed above all checked out. One of them could be hanging closed under regular pressure drops. Then, when you drain the tanks, the pressure differential is larger which causes it to open. Once pressure is built up, the differential is less and it hangs closed, until you drained the tanks again. Just a theory of course...but there have been problems with them before.

This is what they look like. Part# KN-23000 is a pretty common number for them and should easily cross if you need to. They're a very simple valve, block off plate and spring, but they can stick.

View attachment 570469

One is on the driverside behind the step as seen in the picture. That is the air filter canister on the left in the photo for reference.

View attachment 570470

The other two are on the forward end of each tank on the passenger side behind the tool box. Both can be seen in this picture.

View attachment 570471

The other thing I would do is install an independent pressure gauge before the gauges, valves and tanks. Maybe the govenor is not able to see a pressure drop, which would cause the compressor not to cut in. Just another theory, but checking pressures in other parts of the system might help you pinpoint the problem.

Yes! super nice pictures and explanations. Thank you so very much. I was looking earlier and that is not what I assumed was the check valves. So greatful for those pictures and directions where to find them. I will take all of them down Saturday and investigate. Can they be disassembled or are they replace only items? I at least have to see if they are crudded up Saturday. I bet that is the issue. I just searched and see they are readily available and sort of reasonabley priced. Heck, better off getting them all and be done with it. This daggone truck is so pretty I might as well think of it as a woman. The better they look the more you got to spend to keep 'em up and running! I keep this up and it'll be a brand new air system. New governor on the way.

The HUlk · Jul 15, 2015

If it won't build air with the governor tube disconnected from the unloader, then either the unloader is malfunctioning or the compressor is not making air. My money is on the unloder valve intermitently malfunctioning. When compressors don't make air odds are it's not an intermitent thing.

The HUlk · Jul 15, 2015

I also notice you mentioned your unloader "was a beast to put back together"? Why so? I have the A1 Cummins 250 and re-assembly was quite simple for me. Perhaps the difficulty is an indicator that a mistake was made in re-assembly? Do you have a diagram to confirm parts placement? I remember noticing there was a large and small spring and thin spacer. They have to be installed correctly for reliable performance. I also remeber when cleaning the unloader top valve seat that there were rubber seals in there. Mine cleaned up and looked new. Did you notice the condition of these?

Ford Mechanic · Jul 15, 2015

I think there it's a check valve on the wet tank also, that's the main one that normally gets so crudded up. But I may be wrong. I think all the tanks have that valve on them. You can try cleaning it, I don't know if it'll help or not. Ornery threads I've read about them said best to replace for normal maintenance.

Ford Mechanic · Jul 15, 2015

The HUlk said:
I also notice you mentioned your unloader "was a beast to put back together"? Why so? I have the A1 Cummins 250 and re-assembly was quite simple for me. Perhaps the difficulty is an indicator that a mistake was made in re-assembly? Do you have a diagram to confirm parts placement? I remember noticing there was a large and small spring and thin spacer. They have to be installed correctly for reliable performance. I also remeber when cleaning the unloader top valve seat that there were rubber seals in there. Mine cleaned up and looked new. Did you notice the condition of these? If they leak then a large percentage of the air made by the compressor will escape through the unloader valve while charging the system.

If he has the 8.3 they are harder to do due to clearance work the IP.

The HUlk · Jul 15, 2015

gotcha.

gottaluvit · Jul 15, 2015

The HUlk said:
gotcha.

Yes. A2 with the 8.3. Horrible engineering.

gottaluvit · Jul 15, 2015

Swamp Donkey said:
There are 3 one way check valves on your truck. Every tank has one except the wet tank. I would check those out assuming the things you listed above all checked out. One of them could be hanging closed under regular pressure drops. Then, when you drain the tanks, the pressure differential is larger which causes it to open. Once pressure is built up, the differential is less and it hangs closed, until you drained the tanks again. Just a theory of course...but there have been problems with them before.

This is what they look like. Part# KN-23000 is a pretty common number for them and should easily cross if you need to. They're a very simple valve, block off plate and spring, but they can stick.

View attachment 570469

One is on the driverside behind the step as seen in the picture. That is the air filter canister on the left in the photo for reference.

View attachment 570470

The other two are on the forward end of each tank on the passenger side behind the tool box. Both can be seen in this picture.

View attachment 570471

The other thing I would do is install an independent pressure gauge before the gauges, valves and tanks. Maybe the govenor is not able to see a pressure drop, which would cause the compressor not to cut in. Just another theory, but checking pressures in other parts of the system might help you pinpoint the problem.

Well, I just ordered 3 new ones. If the wet tank does indeed need one I will eliminate the other three first and then for sure one of the original check valves has to be good and I can find out which by trial and error testing on the wet tank with the three I just changed out.

Jim

gottaluvit · Jul 15, 2015

Ford Mechanic said:
I think there it's a check valve on the wet tank also, that's the main one that normally gets so crudded up. But I may be wrong. I think all the tanks have that valve on them. You can try cleaning it, I don't know if it'll help or not. Ornery threads I've read about them said best to replace for normal maintenance.

Oh well for now. Already ordered three. See previous post

The HUlk · Jul 15, 2015

gottaluvit said:
Oh well for now. Already ordered three. See previous post

When in doubt, change em all out. That's how I do.

Swamp Donkey · Jul 15, 2015

Ford Mechanic said:
I think there it's a check valve on the wet tank also, that's the main one that normally gets so crudded up. But I may be wrong. I think all the tanks have that valve on them. You can try cleaning it, I don't know if it'll help or not. Ornery threads I've read about them said best to replace for normal maintenance.

I'll take your word for it, as it's been a minute since I checked mine, but I seem to remember there only being 3. None of the ones pictured are on the wet tank. If I missed one I'd like to know about it.

gottaluvit said:
Yes! super nice pictures and explanations. Thank you so very much. I was looking earlier and that is not what I assumed was the check valves. So greatful for those pictures and directions where to find them. I will take all of them down Saturday and investigate. Can they be disassembled or are they replace only items? I at least have to see if they are crudded up Saturday. I bet that is the issue. I just searched and see they are readily available and sort of reasonabley priced. Heck, better off getting them all and be done with it. This daggone truck is so pretty I might as well think of it as a woman. The better they look the more you got to spend to keep 'em up and running! I keep this up and it'll be a brand new air system. New governor on the way.

Yes they can be disassembled. I agree with replacement though. I got mine at Truck Pro for $12 a piece without an account.

Ford Mechanic · Jul 16, 2015

http://www.steelsoldiers.com/showthread.php?81288-m939-air-system-check-valve

Check this thread on check valves, it also said 3 valves, I'll check my truck tomarrow and see how many I see. This also has part numbers for that valve.

gottaluvit · Jul 16, 2015

Swamp Donkey said:
I'll take your word for it, as it's been a minute since I checked mine, but I seem to remember there only being 3. None of the ones pictured are on the wet tank. If I missed one I'd like to know about it.

Yes they can be disassembled. I agree with replacement though. I got mine at Truck Pro for $12 a piece without an account.

View attachment 570494

I think that's a different style. I didn't go as deep in mine as one could have for fear of complete screw-up. I like the idea of r and r on a 12 dollar part anyway. I got mine for that too with free ship.

Air Fails To Build Intermittently; Parking Brake?

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