Air Issue Sometimes

[Melbo]

I seem to be having an air issue.  I'm not sure I can quantify it exactly.  The bus airs up normally most times.  After it is aired up when I shut it off I don't hear any air leaks.  Sometimes it seems to take longer than normal to air up so I will turn it off to see if I hear any air leaks and there never are any I can hear.  Sometimes when I am driving and stop at a light or have to wait with my foot on the brake the air pressure seems to drop BUT most times it doesn't.  I have run the air down with the brakes and watched it rebuild and it seems to do that like it should.  When driving  on the highway it runs up to 120 then drops to just over 100 and then runs back up in a normal cycle.  Just looking for pointers on what I might be missing or where I might check.  I has been about 10 years since I replaced the regulator but when I did that it was obvious that it was not working correctly.

Thanks

Melbo

[luvrbus]

Unloader valves are starting to carbon up would be a good guess

[Melbo]

Thank You Clifford,

Is that a repair or replacement situation.  It looks like I have a Bendix air compressor so it should not be hard to source one and just replace it. It is a lot more accessible than the one I did on the 8V71. 

Melbo

[Dave5Cs]

NAPA has them in a kit with two that you will need. Depending on which compressor you have. They are easy to replace. Two bolts 7/16 IIRC on top where the air inlet from horn comes to the top of compressor. They are inside that. Push up on them with needle nose pliers and pull them out reverse to put in the new ones.

[luvrbus]

Pull the discharge line if it's getting oily it is only a matter of time before you need a compressor and if you have a separate air cleaner for the compressor check it   

[chessie4905]

As Clifford says, disconnect compressor outlet line. Also if it has a ping tank or muffler, drain it to see if you are getting significant oil. Have you been getting oil at the air tanks or drier if you have one? Keep in mine that an air compressor is like an engine. Any oil getting past the rings goes to the air output instead of into exhaust system like a car.

[Melbo]

Clifford

Yes I do have a separate air filter and that was the first thing I checked.  I will pull the discharge air hose and see what kind of air I have there and if it is oily.  If I need to replace the compressor it needs to be timed when installed. It is also necessary to remove the fuel pump.  I'm hoping that it is a repair per Dave but I will soon know and I will update.

Thanks for the info

Melbo

[Jim Blackwood]

Time the air compressor? Never heard that before. Is that for vibration control?

Jim

[luvrbus]

Mel has a Cummins engine with the PT pump they do need to be timed the pump is driven off the compressor 

[Jim Eh.]

Everything in a row ...

[Melbo]

Well I pulled the discharge line and it is not oily and seems to have pretty good air pressure.  The unloader parts for a TU-FLO 700 bendix seem to be readily available however I am unable to find very specific instructions for the replacement of the parts.  Dave's instructions are about what it says in the manual.  I guess I will investigate further. Any hints or additional advice is greatly appreciated.  The compressor is very accessible so I think it would be a pretty basic procedure.  I have one more test I want to do to be sure that the recovery is within the parameters for a bendix 700 series.

Thanks for all the help

Melbo

[Melbo]

I took the bus out for a cruise yesterday.  It aired up like normal and I did an air recovery test per the Bendix spec sheet and it recovered just fine.  Took it to the truck wash and fuel stop and filled it up with water getting ready for a trip to Rocky Point.  Then I did the air recovery test again after it was all warmed up.  The air kept up and operated normally when driving but when parked it was a little slower than the Bendix specs. 

The air compressor is supposed to recover from 85psi to 100psi in 40 seconds.  When I first started it was close to 30 seconds but after driving about and having it all warmed up it was closer to 50 seconds. 

So here is my question.  What would cause the recovery rate to change.  Is it the unloader valves as Clifford mentioned or is there something internal so the compressor needs to be replaced.

Just looking at my options here.

Thanks

Melbo

[Dave5Cs]

Unloader valves or D2 air governor is possible also. Might replace it since they are only about 12.00 at NAPA. I keep 2 in the bus in case I need them.

[buswarrior]

You are doing the test by airing the coach up all the way, then reducing pressure via the brake pedal down below 85 and start timing as it comes up past the mark, to conduct the test?

Parking brake in the same configuration for both tests?

Governor failure usually is found to make cut-in and cut-out erratic or non-existant.

happy coaching!
buswarrior

[Melbo]

Dave  Thank you.  I see now that the unloaders are just behind the supply air connector on the compressor.  I did not see that until I got out the exploded view.  I can be a little slow sometimes. 

Yes BW I get the pressure up to over 100psi when I have the parking brake set.  I then press the brake to reduce the pressure to 85psi and turn on my timer and watch for the pressure to reach 100 with the engine at idle speed. 

When driving the gauge goes up to 120 then drops and just over 90 psi starts to climb and goes up to 120 and repeats the cycle. 

The issue seems to be when I am on and off the brakes a lot.  When I'm backing into storage and stopping and checking my mirrors and moving a little and then stopping and moving a little more and stopping etc.  I was wondering if I had an air leak at the treadle valve but I don't hear anything unusual. When I shut the bus down I don't hear any air leaks. I'm just trying to stay ahead of maintenance that may need to be done and to stay safe with the brakes.

Thanks

Melbo