Author Topic: MCI 9 bus air  (Read 4089 times)

Offline jatnip

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MCI 9 bus air
« on: May 07, 2006, 10:38:14 PM »
I have a 1979 MCI bus and twice on start up the air will build up to about 90# and then will fall back to about 60# and stay there.  Once it was in cold weather and after releasing air with break pedel and building back up after several times it started working normal again.  The other time it was warm out and right after power washing engine compartment it did it and after releasing air with break pedel once it started working normal.  Any thoughts? Thanks in advance.

Offline FloridaCliff

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Re: MCI 9 bus air
« Reply #1 on: May 08, 2006, 04:49:28 AM »
Hook up some shop air and get out the soapy water spray bottle.

With the shop air hooked up walk around and listen closely, chock the wheels and do the same with the brake released, have someone push down the brake while you listen at each brake. 

I had an air bag that stopped leaking(for a week) after I pressure washed near the bags, go figure.

Just a starting point ;D


1975 GMC  P8M4905A-1160    North Central Florida

"There are basically two types of people. People who accomplish things, and people who claim to have accomplished things. The first group is less crowded."
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Offline gumpy

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Re: MCI 9 bus air
« Reply #2 on: May 08, 2006, 06:47:12 AM »
Probably your air compressor governor is sticking.

Had the same symptoms. Tapped the governor and it freed up and worked. Rebuilt the governor. No problems since. I carry a spare governor, now.

Craig Shepard
Located in Minnesquito - "Some Assembly Required"

Offline darrenayres

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Re: MCI 9 bus air
« Reply #3 on: May 08, 2006, 07:04:18 AM »
Most MC-9s have four air tanks. The wet tank fills first then the dry air tank, parking brake tank, and finally the accessory and air ride tank. Each tank has a valve that closes the release of air until it reaches about 90 lbs. Thats why your air pressure guage seems to stay at 90 a long time before moving upward again.  Generally, the compressor will not shut off until the accessory and air ride tank reaches the 120# mark.

I'm thinking off the top of my head. I'm not a mechanic but I've had three MC-9s and have had different problems with the air systems. I also have one of those coveted Crusader II Maintenance Manuals, original!  To narrow things down, start pulling the drain valves on each tank when it reaches 90 lbs. You can identify how far along the system is holding air. When you get to the tank that is empty, you know there is a leak between it and the last one. If a leak is the problem.

Do you know where the tanks are?