Not building pressure in main air tank

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Turkrypty

New member
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Hebron CT
So I started my Mog up today and waited for it to warm up and build pressure, but it never did. The gauge showed 45lb but would not build. I shut down the machine and investigated. i drained the air from both tanks and found that air was getting to the small rear tank, but not to the larger main tank right next to it. there was a significant amount of air coming out of the bottom of the alcohol injector (set to summer mode) and the truck did its typical "Psst" air purge, but the main tank, and gauge never rose. None of the hydraulic actuators worked either.

Anyone have experience with this?
Im going to start looking trough the TM's but that might take a while.
 

peakbagger

Member
525
19
18
Location
northern nh
A trick that several other members have done is backfeed the system with compressor so that the sound of the engine does not overwhelm where the air leak is. There is a technical advisory on the cross body air line that goes from the back side of the frame rail where the pressure regulator attaches to the air tanks. Its a low spot n the system and tends to rot out while stored. The technical advisory is to replace the line with stainless. One of the members on the forum is in the final stages of making one for sale but not sure if he is done. Some of the SEE pressure regulators appear to have an internal screen that can get clogged but its on the inlet so I dont think the spit valve would be working. My standard advice with SEEs is start at the alcohol regulator and start cleaning out the rust in the system. That cross body air line rots from the inside out and will throw a lot of rust flakes into the downstream side of the air system. One warning with the air system is the combination of the CARC coating and the fittings material of construction is care must be taken when taking the fittings apart. A collection of properly sized tubing wrenches and wrenches is essential, don try to use a crescent wrench in place of the correct size wrench. You need to "backup" the fittings with a wrench as if you apply torque to the actual fittings they may break. There is mix of 2 possibly three thread types. You will find varying amounts of rust in the system and the air tanks. If you have the right tools (time to visit harbor freight for a jumbo wrench set). Its about a 3 hour project. If as you progress the amount of rust is still significant realize there is lot of complex air controls up under the cab.
 

peakbagger

Member
525
19
18
Location
northern nh
A trick that several other members have done is backfeed the system with compressor so that the sound of the engine does not overwhelm where the air leak is. There is a technical advisory on the cross body air line that goes from the back side of the frame rail where the pressure regulator attaches to the air tanks. Its a low spot n the system and tends to rot out while stored. The technical advisory is to replace the line with stainless. One of the members on the forum is in the final stages of making one for sale but not sure if he is done. Some of the SEE pressure regulators appear to have an internal screen that can get clogged but its on the inlet so I dont think the spit valve would be working. My standard advice with SEEs is start at the alcohol regulator and start cleaning out the rust in the system. That cross body air line rots from the inside out and will throw a lot of rust flakes into the downstream side of the air system. One warning with the air system is the combination of the CARC coating and the fittings material of construction is care must be taken when taking the fittings apart. A collection of properly sized tubing wrenches and wrenches is essential, don try to use a crescent wrench in place of the correct size wrench. You need to "backup" the fittings with a wrench as if you apply torque to the actual fittings they may break. There is mix of 2 possibly three thread types. You will find varying amounts of rust in the system and the air tanks. If you have the right tools (time to visit harbor freight for a jumbo wrench set). Its about a 3 hour project. If as you progress the amount of rust is still significant realize there is lot of complex air controls up under the cab.
I has to chase down a complete loss of air event on my SEE today after losing my air system last weekend when I was about 45 minutes away from home. I had done some quick checks when it happened but headed home without brakes. I had zero air pressure on the gauge and no air in the primary tank but I could hear the spit valve running. I decided to try the backfeeding the gladhands but I didnt have much luck. I have never had a use for trailer brakes and have ignored the system. Turned out I was missing one of the blanking devices that keep the gladhands sealed when nothing was hooked up to them. I also had one of gladhands with a lot of corrosion under the sealing rubber. I ended up just removing it and hooked an air fitting to the tubing. Incidentally the red needle on my dash has never worked. Maybe the gladhand trick worked for others but it didnt work for me on the primary air system. It turns out each air tank is fed by a backpressure control valve so in theory one tank can not feed the other one. I switched back to diagnosis mode and eventually after eliminating a lot of things I got it fixed. I wish I had a hard and fast cause but I I think it was a plug in the back pressure control valve that feeds the primary air tank. I cant be sure, as removing the valve requires disassembling it unless the air system is close to completely disassembled. When I pulled the drain plug on the primary tank I found a lot of large "crunchies" that were too big to go through the tank bleeders. I didnt find any crunchies in the primary bleeder that sits out back. There is not a lot to these pressure control valves, just a diaphragm and a spring assembly. They really act as a positive check valve to keep the air in the tanks and to keep one from backfeeding another. My diaphragm seems to be intact and when I reassembled the systems things started working.

I did some searching for these valves in case I needed one and they look like they are not readily available. There are similar ones out there with different thread configurations but given the close quarters it may be major challenge to get anything to fit. The best option may be to try to find suitable material to replace the diaphragm.

By the way, as usual working on the air system is a major PITA, its rare to be able to take one component off without having to remove other pieces of tubing. Some of the fittings are very "tender". The CARC coating means getting a good wrench to fit is tough. Do not even try to use Crescent wrenches.
 
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