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nicklepickle

Not Building Pressure

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So, i have a 1974 MB400 with a Scania engine. Last winter the head on my Tu-Flo 501 air compressor cracked. I had the compressor head replaced, then started it up. i build around 30 PSI but nothing more than that. Assuming it was the regulator, I took it off to check. It would build up to 60 PSI, and i could rev it beyond that, but it would always drop back down to 60. I then bypassed the air dryer and routed it directly into the tank. Still nothing more than 60 PSI. So, I had the compressor rebuilt, put it back in yesterday without the regulator and now it won't build any pressure at all. I can feel air coming through the output of the compressor, and can follow it all the way to the tank. There is a small leak in the parking brake release in the cab, but it doesn't seem big enough to be taking all the pressure away. Don't really know what to do now.

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I will admit I don't know much about the compressor but did you look to see if the intake was clogged? You may also have a valve sticking or worn out. I know that the valves will carbon up and stick. Just some things to think about.

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I will admit I don't know much about the compressor but did you look to see if the intake was clogged? You may also have a valve sticking or worn out. I know that the valves will carbon up and stick. Just some things to think about.

The in take is fine. It got cleaned and a new filter was put on when the thing was rebuilt. All the valves should be fine also.

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The in take is fine. It got cleaned and a new filter was put on when the thing was rebuilt. All the valves should be fine also.

Remove the exhaust piping from the compressor and ensure it is pumping air. If not, valves that "should be good", may not be. New is not always good. If the compressor is not pumping air from the exhaust, it cannot pull air in the intake. This would be a case of the valves not sealing, or installed incorrectly.

An air compressor is nothing but a pump that elevates atmospheric pressure to higher than ambient when the discharge is exhausted into an air receiver, (tank). It will not build pressure if the exhaust is open, but rather pass the air through.

My vote is that you either have an open in your discharge line, massive leak, or mechanical deficiencies with your compressor rebuild. The truck running is too noisy to hear an air leak. Plug shop air into the "wet tank", (first tank compressor discharges into) and listen for air leaks. If none heard, spray a soap bubble solution over all connections and lines. You will either find the source of your leak, or prove the compressor needs rework.

Rob

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Rob has the problem solved air it up from the compressor end and see what happens. Also there maybe a check valve in the first tank from the compressor that is carbon up. The way to check the air governor to see if it is bad is unbolt from compressor and it will pump if governor is bad or if it ant on the compressor then take the line off to the compressor and it should pump air if bad. But by all means air up first and that way you want be guessing and buying parts that is not bad. Keep in mine there still can be a problem with the compressor. glenn

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Ever hear of running shop air back into the compressor itself through the discharge line, building it to 100 PSI, and listening for leaks as a way to discover a open discharge valve? I'm going to try both this and running air into the tank. At least then I'll know which end is which.

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Ever hear of running shop air back into the compressor itself through the discharge line, building it to 100 PSI, and listening for leaks as a way to discover a open discharge valve? I'm going to try both this and running air into the tank. At least then I'll know which end is which.
There may be a check valve between the tank and compressor. At one time trucks had that but some dont use it.

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First thing I would do is take the discharge of the compressor loose to make sure that unit is moving air through it. If it is, reconnect, and if there is a check valve, remove it and verify that it is not stuck disallowing free flow into the receiver, (wet tank).

Rob

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Well, I ran shop air into the tank, and capped the line back into the compressor just to isolate it from the rest of the system. I found out i have a leak somewhere past the wet tank, but i can't figure out where. So unless anyone else has a better idea, I'm going to try and isolate each of the lines, pressurize them, and see which ones leak. by the way, there is air coming out of the compressor discharge, and I can feel, with the air filter removed that the compressor is sucking air in, so I'm pretty sure everything is fine with the compressor itself. However, if anyone knows off hand where the air pressure gauge reads from it would be helpful. Otherwise I'll trace it tomorrow.

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Well, I ran shop air into the tank, and capped the line back into the compressor just to isolate it from the rest of the system. I found out i have a leak somewhere past the wet tank, but i can't figure out where. So unless anyone else has a better idea, I'm going to try and isolate each of the lines, pressurize them, and see which ones leak. by the way, there is air coming out of the compressor discharge, and I can feel, with the air filter removed that the compressor is sucking air in, so I'm pretty sure everything is fine with the compressor itself. However, if anyone knows off hand where the air pressure gauge reads from it would be helpful. Otherwise I'll trace it tomorrow.

Look on your tanks for a 1/4" plastic line. It could be picked off of a wet, or dry tank, or a manifold mounted to the cab. I don't know a thing about an MB series truck as far a layout.

Rob

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It sounds like your problem from the start was an air leak or a few air leaks,I don't understand why you can't find it,start at the compressor and work your way through the whole truck spraying every line and valve with soapy water use dish soap it works the best.Good Luck

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Macks, you were right. It ended up being a bad fitting on the low pressure indicator way up in the front of the cab and a burned out o-ring in the brake release , so I couldn't hear it. Replaced it today, $50 for a piece of plastic, a spring, and a watch battery, and everything works fine. And i have a newly rebuilt compressor that I didn't need.

thanks all.

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That the reason you have to do some testing or checking or otherwise you are only guessing. glenn

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