Air Pressure not Holding

[Frank @ TX]

I have an MC9 , 1984.
We converted it in 2001.
For as long as we've had it the main pressure would stay at about 60 PSI for about a month in storage.  Now the pressure drops to 0 PSI in about a week.  Also I've noticed during our winter trip the compressor would unload every 5 MIN instead of 30 MIN like before. ( yes I time these kind of things )  also when stopped at a fuel station when I start it up I need to let it run for a while to build air before I could go again.  There must be a big leak somewhere.  So far I checked the AUX tank and the systems run from it. The AUX tank is tight and no leak at the systems.
I remember about a pressure protection valve somewhere that would not let the air get lower than 60 PSI.
Is there anything else I should be looking at too ?

Thanks for your help
Frank

[robertglines1]

most common   : air bags     leveling valves    lines      soapy water and spray bottle time. look for a air line that might be rubbed into.  good luck

[bevans6]

I would also suspect the suspension.  the pressure protection valve is in between the brake system and the rest of the system, so if you drop fast to 60 and then it holds for a while, the drop is in the aux or parking brake systems.  If the pressure drops evenly right past 60, either the PPV is bad or the leak is in the main service brake system.  The reason I sort of suspect the suspension is that it's the biggest user of air while travelling, the leveling valves are constantly adding deleting little bits of air as the bus sways on down the road.  Not much normally, there are delays built into the system, but if a leveling valve fails I can see the thing calling for air pretty heavily.

MC9, the dash gauge reads the pressure in the dry tank,  which is the main service brake supply tank.  BTW, your bus seems pretty tight, those are excellent numbers - except for cycling every 5 minutes on the road!  that means it's losing 30 pounds in 5 minutes, or 6 psi a minute, which exceeds the allowable  DOT air loss.

Brian

[Oonrahnjay]

Frank, you need to check your "immediate" pressure drop.  Air it up to 120 lbs., cut off the engine and check how much it drops in half an hour.  The DOT maximum is 60 lbs. (you could also check 15 minutes with max drop of 30 lbs, etc.; you're also allowed a drop of 3 lb/minute with the engine not running and the brake pedal held down).  I'm going to guess with what you've said, you don't have a *big* leak -- but it's a leak that's bigger than I'd be comfortable with.  As Robert said, nothing you can do except get under it and check lines, fittings, components, etc.  But I'd be surprised if it isn't a fairly small, gentle leak so it may not be easy to find.

Also, the "protection" valve is there to isolate the "non-critical" systems from the main air brake system.  When the engine is running, the air brake system is filled first, then the protection valve opens at 60 lbs and from then on the compressor is filling both systems.  After you shut down, if the "non-critical" part of the system is leaking, the entire system drops to 60, then the protection valve closes; from that point on the non-critical parts continues to leak down but the air brake part of the system leaks at a slower rate (assuming that it doesn't have leaks itself).  Of course, it's hard to tell on the InnerWeb, but the way that your bus acts (i.e. dropping pretty quickly in the "upper pressures" while the bus is running and then dropping at different rates when the engine is shut off) is pretty good evidence that a leak is in the "auxiliary" area -- but you never know until you do a complete and thorough check.

If you have an air system chart, you should be able to tell which pressure gauges indicate which area of the system.  By the way that the different areas of the system drop at different rates, you can get an idea which part (s) of the system has the leaks.  Note, there are one-way valves in the many systems, you'll need to account for them as you see what parts of the system hold air and which parts are dropping.

Every one of us who owns a bus should be 100% familiar with all the components of the air system -- if we are, we'll know which part of the system is attached to release valves, particular runs of tubing, etc.  Then when we find a leak in one place by using the old tried-and-true procedure, we'll know immediately which of the systems is affected, how that translates into other areas of the system, etc.

As Robert says, look at air bags, leveling valves and tubing but don't forget to check other items such as valves etc.  Often protection, regulation, application  (the ones under the brake pedal), and similar valves have balances or relief ports to the outside air.  If there is a problem inside the component, you may get air leaking from those ports -- that's a sign of an internal failure.  Also, a small leak may be in the miniature line to the pressure gauge.  If you're not sure, remove the fitting from the tank that leads to that little piece of tubing and put a plain metal plug in it.  If the pressure drop changes, it shows that the leak is in that part (i.e. the tubing, connectors, pressure gauge itself, the warning horn switch, etc.)

[buswarrior]

Hello Frank.

As above, and...

Check the exhausts on all the brake valving for seeping air.

the relay valve and inversion valve down the back, the treadle valve under the driver.

Chock the wheels and try it both parking brake applied, and parking brake released.

And with the brake pedal released and applied.

Check the exhausts and where the lever enters the body of the leveling valves.

Isolate the shutter system via the shut off valves on the filter device.

Oonrahnjay, which jurisdiction has that 60 lbs in an hour rating?

happy coaching!
buswarrior

[TedsBUSted]

Don't forget the  the compressor discharge line check-valve and the governor.
They can both have sizable leaks that are rather muffled.

[robertglines1]

you checked aux tank-Did you spray around bottom(sometimes they get pin holes-many) Take floor access out and spray all hoses and filter housing in sight. good place for hoses to rub on each other or metal. Also don't forget you tag axle lift pancakes-they seem to deteriorate because they are seldom used. Shop air or auxiliary air the system so you can here over bus running.

[Frank @ TX]

Thanks guys ,
there is NO air in the dry tank but the air bags are still up.
Early on I put a check valve between the aux tank and the main system.
So I'll just keep looking and spraying.
Frank

[robertglines1]

with ck valve separating systems--parking brake valve   foot valve   hole in parking brake diaphragm   air dryer   wet tank drain   

[Oonrahnjay]

with ck valve separating systems--parking brake valve   foot valve   hole in parking brake diaphragm   air dryer   wet tank drain   

Yes, you've narrowed it down to one part of the system and that's good.  It's also good that you're not dealing with the stuff on the auxiliary side (horn, air suspension, air ride seats, air throttle, air operated door opener, etc.) -- all that stuff can be a pain in the backside.  The only downside is that the system that you do have to trace down is the safety-critical portion!

It sounds as if you don't have protection valve between your brake components and the auxiliary side.  You might want to check on adding one there.  It provides for quicker build of the pressure on the brake side and protects the braking system if you have a big leak on the auxiliary side. 

Also, I went back and corrected the time to leak down.  It's 60 lbs in 30 minute, starting with "fully charged tanks" and the engine off; (also, the same test repeated with the brake applied is 60 lbs in 20 minutes).  Personally, I want to see no more than 1 pound a minute loss; I wouldn't drive my bus with a quicker loss than that -- my bus now leaks about 6 lb in 30 minutes and I know that I have a tiny leak in an old pressure gauge (have a new gauge to replace it).  Sorry about the error before, what do you expect from an amateur backyard tinkerer?

[buswarrior]

Hold on...

Assumptions ranging wildly... time wasting listing of the entire air system.... bonus points for whoever lists the remaining missing pieces...

How about something methodical and strategic so as to not waste hours of time and gallons of soap?

Not to mention, and more importantly, not everyone has limitless amounts of crawling-under-the-coach energy or agility to be fooling around under there.

We need something to aim at.

Ok, I digress...

Holes empty the air system, tired valves seep air.

The leak is too light to be the typical hose rub or rust flake off a tank.

The way this is described, there is seepage. So, the strategy is to check where air usually exhausts, for leaks, when there shouldn't be.

Air suspension staying up confirms the air suspension only, which is good news. The leveling valves do not leak back to the tank, if it empties. It also confirms the leveling valves.

As for the rest of the auxiliaries... is there air in the aux tank?

If the dry tank is down, I expect you will find a brake valve weeping air out an exhaust.

happy coaching!
buswarrior

[Joe Camper]

All these valve have................yes you guessed it, RUBBER COMPONENTS in them and can only be expected to provide a reasonable lifespan.

Everything is suspect, most valves are inexpensive, anytime you see one that is easily replaceable when the bus is 20 yr old just do it.

You gave me a couple of clues that tells me you will find it.

First that you noticed it and second if you are loosing enough air at a fuel stop to delay departure it is leaking bad enough to probably hear it that is a pretty rapid loss.

On a side note about the brake pedal 1 way a bad R-12 valve can fail (a brake relay valve, 15 bucks, always right on the axle, often A SINCH TO PULL) will push air back thru the exhaust port on the brake pedal.

[Joe Camper]

On our 85 I can drain the wet tank leave the valve open.


Both dash gauges hold.

I like that. Is that weird LOL

[TedsBUSted]

Hold on...

. . .How about something methodical and strategic so as to not waste hours of time and gallons of soap?

Yes, maybe put a shot of bacon grease in the compressor line and then turn the dogs loose. 

But actually, if someone wanted to get real serious about the hunt, I wonder if a shot of oderant and a gas sniffer wouldn't quickly pinpoint even the slowest leak.

Ted

[Joe Camper]

You just need to know your bus, want to hold air and persevere