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Air Compressor 81 R Model


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Hello friends I just put a new compressor in my 81 Mack because of oil in lines! I hope I never have to take it off again :blink: . Anyway I drained tanks twice no oil or water now but my air was too slow to build so shut truck of with service brakes on wheels chalked and air finally to 120 psi. I found a leak in front rear tandem air chamber and my question is will one leak cause the air to build very slow? I actually have to throttle it to get it to raise much. Also my spring brakes don't apply fully when fanning the button does not pop out!

Will a new chamber fix it all or just the rate of compression? I also put in some new slacks! I am trying to get this thing back in business.

Edited by woreoutmack
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Hello friends I just put a new compressor in my 81 Mack because of oil in lines! I hope I never have to take it off again :blink: .

Just think, in another 29 years, when you have to change an air compressor on an MP8 in an old wore out GU, you'll look back at this and remember the good ole days. A leaking air chamber diaphragm will keep the compressor from building air. Remember, when appplying the spring brake you are not putting any air into the chamber. If you have the spring brake applied and it's leaking air out of the chamber you need to check further up line to find out what is supplying air to the chamber when the spring brake is set

"Mebbe I'm too ugly and stupid to give up!"

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Just think, in another 29 years, when you have to change an air compressor on an MP8 in an old wore out GU, you'll look back at this and remember the good ole days. A leaking air chamber diaphragm will keep the compressor from building air. Remember, when appplying the spring brake you are not putting any air into the chamber. If you have the spring brake applied and it's leaking air out of the chamber you need to check further up line to find out what is supplying air to the chamber when the spring brake is set

Lol in 29 years I will be happy if I am still alive. I did not have park brake set with them set air builds a little faster but not as fast as I thought a new compressor should! I get conflicting time amounts to build full air in pretrip info does this truck need to make full air in 3 minutes or 60 seconds? The spring brake does not set at 25 psi could that be the chamber as it is on the spring brake chamber? Thanks in advance this truck is my labor savor so I need it up and running!

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Lol in 29 years I will be happy if I am still alive. I did not have park brake set with them set air builds a little faster but not as fast as I thought a new compressor should! I get conflicting time amounts to build full air in pretrip info does this truck need to make full air in 3 minutes or 60 seconds? The spring brake does not set at 25 psi could that be the chamber as it is on the spring brake chamber? Thanks in advance this truck is my labor savor so I need it up and running!

With the spring brake released air is SUPPLIED to the chamber to hold off the spring. If the diaphragmm is ruptured it will allow holdoff air from the spring chamber to pass through to the service side of the chamber, which will exhaust through the quick release valve. This could be the cause of your slow build up time, but even if its not, you need to get it fixed.

"Mebbe I'm too ugly and stupid to give up!"

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With the spring brake released air is SUPPLIED to the chamber to hold off the spring. If the diaphragmm is ruptured it will allow holdoff air from the spring chamber to pass through to the service side of the chamber, which will exhaust through the quick release valve. This could be the cause of your slow build up time, but even if its not, you need to get it fixed.

Yes I know and plan to by weeks end. The truck is out of service until I get it all right I managed to limp home from a job but immediately started looking for the problems. I am trying to figure out why the brake button wont pop off when air gets below 25psi and was wondering if it could be this chamber also causing that. If not I was hoping to learn what was doing it, so; when I take the core back to the dealer, I could pick the chamber and whatever is not automatically setting spring brake at low air! What triggers the park brake button to pop out at low air?

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Yes I know and plan to by weeks end. The truck is out of service until I get it all right I managed to limp home from a job but immediately started looking for the problems. I am trying to figure out why the brake button wont pop off when air gets below 25psi and was wondering if it could be this chamber also causing that. If not I was hoping to learn what was doing it, so; when I take the core back to the dealer, I could pick the chamber and whatever is not automatically setting spring brake at low air! What triggers the park brake button to pop out at low air?

It's probably a bad dash valve. This document from Bendix (http://www.bendixvrc.com/itemDisplay.asp?documentID=4572) explains the function of the dash valve. Basically the dash valve has a spring that causes the inside parts of the valve to move to the exhaust position when air pressure drops below a certain point. If your compressor was pumping oil for an extended amount of time that oil has made it through the air system to the valves and air chambers and will cause problems. The air compressor was probably the root of the problem and since you've replaced it you're done with a good part of the work. Do you have an air dryer on this truck? If the dryer cartridge is contaminated by oil it can come apart and send oilly debris through the system. Good luck.

"Mebbe I'm too ugly and stupid to give up!"

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It's probably a bad dash valve. This document from Bendix (http://www.bendixvrc.com/itemDisplay.asp?documentID=4572) explains the function of the dash valve. Basically the dash valve has a spring that causes the inside parts of the valve to move to the exhaust position when air pressure drops below a certain point. If your compressor was pumping oil for an extended amount of time that oil has made it through the air system to the valves and air chambers and will cause problems. The air compressor was probably the root of the problem and since you've replaced it you're done with a good part of the work. Do you have an air dryer on this truck? If the dryer cartridge is contaminated by oil it can come apart and send oilly debris through the system. Good luck.

Thanks I searched it last night and came up with the same thought that oil fouled the button. No drier I will replace the chamber and brake button and hope that gets it. Thanks oh do you know the torque on the insulator bolts spring pads? I am going to replace them too!

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Just think, in another 29 years, when you have to change an air compressor on an MP8 in an old wore out GU, you'll look back at this and remember the good ole days. A leaking air chamber diaphragm will keep the compressor from building air. Remember, when appplying the spring brake you are not putting any air into the chamber. If you have the spring brake applied and it's leaking air out of the chamber you need to check further up line to find out what is supplying air to the chamber when the spring brake is set

News for ya!The GU won't Be here In 29 years!It will be in the scrap beside all the other throw away Trucks that are being built today!

But having said that ,Yup remeber what ya hope for!Cause a Any RD CH or older truck Even a RS tandem tandem steer is easy compared to a GU count your self lucky.

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News for ya!The GU won't Be here In 29 years!It will be in the scrap beside all the other throw away Trucks that are being built today!

But having said that ,Yup remeber what ya hope for!Cause a Any RD CH or older truck Even a RS tandem tandem steer is easy compared to a GU count your self lucky.

Lol I am just a wimp and that dern thing was heavy but I got it in and oil line too thanks to ya'all I searched and learned how many thanks to you guys for sharing :thumb:

Edited by woreoutmack
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I want to thank you guys I have it going right again. I even replaced the broken emblems and going to rework the cap with a fiberglass repair kit then shoot some paint will post pics when I do.It has a hard life especially before I bought it all body damage done by prior owners! Anyway this is what it does for me!

002-4.jpg

Edited by woreoutmack
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Also you should check your alcohol bottle for oil if that old compressor was dumping oil into the system, you don't want oil to be left in it and then keep being put through your system. For the truck to build to full air pressure in 3 minutes isn't that bad IMHO. I think the spec is that if it is a dual air system (with a primary and secondary tank) that it should build from 85 to 100 with the engine at operating rpm... so if your truck is getting from no pressure to operating pressure inside of 3 mins, it should be legal.

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When you changed the compressor did you check the cooling lines for restrictions. I'v seen them rusty and blocked behind the pump. Also was the compressor pulling air from the negative presure side of the turbo? Mack had a service bulletin about moving this hose to the intake manifold at the rear of the engine or installing a filter at the compressor. Low/no cooling or restricted intake can shorten the life of your compressor.

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When you changed the compressor did you check the cooling lines for restrictions. I'v seen them rusty and blocked behind the pump. Also was the compressor pulling air from the negative presure side of the turbo? Mack had a service bulletin about moving this hose to the intake manifold at the rear of the engine or installing a filter at the compressor. Low/no cooling or restricted intake can shorten the life of your compressor.

Yeah it is going in between filter and turbo I will get a filter kit soon. Good waterline flow!

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