Funny air leak question.

[bevans6]

My bus was inspected for it's annual last week, and had a funny air leak that has self corrected.  I would appreciate thoughts on what it could have been.  MCI, has an air dryer, then wet tank, then dry tank, with the normal check valves between the dry and wet tanks, and the wet tank and the air dryer.  Symptom was it aired up, purged, air kept leaking from the exhaust valve of the air dryer, and gauge pressure dropped from 120 to 90 psi in about 5 minutes (obviously that rate of leakage is a DOT failure) and then stopped.  This morning when I went to the shop to discuss it, the leak was gone so the bus passed inspection.

It might never reoccur but I want to make sure I understand it.  The first and perhaps most important question is does anyone know with certainty whether the dash air gauge in a 1980 MCI reads the dry tank or the wet tank.  I have always assumed the dry tank, but it's a whole lot farther from the dash (just by the rear axle) and the wet tank is in the front axle bay.  The wet tank should read the same air pressure as the dry tank since there is a one-way check valve that passes air from the wet tank to the dry tank but not vice versa.  In this case, when the governor signaled the air dryer to purge, the air inside the air dryer should have exhausted but the check valve from the wet tank should have retained air inside the wet tank.  It obviously leaked and air exhausted out the purge valve until air pressure reached the cut-in pressure, at which point the purge valve was closed by the governor.  The question in my mind is - would the gauge have shown air pressure dropping in just the wet tank, presuming the check valve to the dry tank worked, or did pressure drop in both the wet tank and the dry tank indicating not one but two faulty check valves?

Thanks for your thoughts on this.  When I get the bus home I will devise a way to test the check valves in situ.  I think that if I drain the wet tank and see what the pressure on the gauge is, then check for pressure in the dry tank, that will both tell me which tank the gauge is reading and if the dry tank check valve is working.  If it's working it will hold full pressure with an empty wet tank, and if it isn't working it will drain with the wet tank.

Brian

[thomasinnv]

Your purge valve was stuck. Probably had some junk stuck in it. Clean/service the valve.

Sent from my SM-G920T using Tapatalk

[Iceni John]

Does your air dryer's purge sound the same as before?   Have you had any cold weather recently that could temporarily freeze something until it thaws later?

For the last few years my AD-9's purge cycle has gradually changed from being a brief PSST to a longer PSSSSSSSST.   I cleaned the purge valve and replaced its O-rings, but it made no difference, and I changed the governor to a brand-new D-2, but also no difference.   I'm thinking that either the 3/4" check valve on the wet tank's inlet is sticking, or the unloader valve in the air compressor is sticking.   Neither is easy to access, so I may just install another check valve immediately after the AD-9 and see if that helps.

John

[bevans6]

My understand is that the purge valve remains open all the time between governor cut-out 120 psi) and governor cut-in (90 psi).  It opens when air pressure is applied by the signal line (governor to air dryer) and closes again when air pressure is removed to close the purge valve and activate the compressor unloader valves.  So if the purge valve had stuck, it would have stuck closed, and it was correctly open instead.  It was supposed to exhaust air, and exhaust air it did.  As soon as air pressure dropped to 90 lbs the governor shuttled and activated the compressor and the purge valve closed.  The question is where did the air come from?  Wet tank, dry tank or both?  I expect I will know tomorrow when I get the bus home.

Brian

[windtrader]

Brian,

I'm not adding anything to help other than the obvious based on what you posted. That valve is clearly not working properly and letting air out when it shouldn't. I had something similar happening to my tire stem valves. Tires holding air just fine, then I check the pressure and the next day the tire is losing air. The valve was not closing due to some particulates. Fully opening the valves cleared things out and now working fine. Maybe you can really work that valve a few times to ensure nothing is clogging it.

Since wet tank is mentioned here, can you folks confirm the location of them for me. I see there are two tanks between the front tires, each with a valve on the side. I have pulled the ring on both and don't see anything coming out with the air. Is there some other place I am supposed to be clearing the wet tank?  Thanks

[TomC]

As strict as California is, they require NO annual inspection or NO smog requirement for older buses and motorhomes. Strange...

[windtrader]

Strange? Other words work better for me - "Wonderful" "Amazing" Gift from God" LOL

Fact is there are so few of us CARB gets far better value and ROI harassing others.

[Jim Eh.]

My understand is that the purge valve remains open all the time between governor cut-out 120 psi) and governor cut-in (90 psi).  It opens when air pressure is applied by the signal line (governor to air dryer) and closes again when air pressure is removed to close the purge valve and activate the compressor unloader valves.  So if the purge valve had stuck, it would have stuck closed, and it was correctly open instead.  It was supposed to exhaust air, and exhaust air it did.  As soon as air pressure dropped to 90 lbs the governor shuttled and activated the compressor and the purge valve closed.  The question is where did the air come from?  Wet tank, dry tank or both?  I expect I will know tomorrow when I get the bus home.

Brian

In your first post you said " It obviously leaked and air exhausted out the purge valve until air pressure reached the cut-in pressure, at which point the purge valve was closed by the governor." The valve is not closed by the govenor pre se, but rather it is opened by pressure. Absense of pressure allows the spring to close the valve.

Same as when you said "It opens when air pressure is applied by the signal line (governor to air dryer) and closes again when air pressure is removed to close the purge valve and activate the compressor unloader valves." Again the activation is by spring pressure and ABSENCE of air pressure.

The unloader port is pressurized when the system pressure from the "wet" or supply reservoir tank reaches 120 (or whatever the governor is adjust to) and THEN pressurizes the unloader port which in turn signals the compressor to go into unload stage and at the same time signals the purge valve in the dryer to allow system pressure to flush the desiccant cartridge. The purge valve should remain closed once the flush cycle is complete. Different air dryers accomplish this a little differently. Ones without a "flush reservoir" use system pressure to "back feed" down the line and clean the desiccant cartridge. there are dryers that use a remote tank which the sole purpose is to supply pressure for a purge and is not connected to the main air system.

If you are running an AD9 my guess is, as stated, your purge valve needs servicing. The other possibility is that the line from the "wet" or supply reservoir tank could be partially blocked in such a manner to allow an unloader to work but artificially keeps pressure in the line due to a blockage or a kink that only allows the line to bleed off slowly.

BTW, to test the check valves in the system, start with all air tanks fully charged. Open the drain on the tank closest to the air dryer first, then the next one down the line, and so on. Each time you open the drain that tank should be fully charged as the check valve is supposed to prevent air from traveling upstream to the tank you previously drained.

The pressure gauge(s) should only connected to the primary and secondary tanks (if dual pressure gauges exist) and not to the "wet" or supply reservoir tank.

[bevans6]

I wrote a thing but it didn't post.  So...

The typical MCI of that era has the wet tank on the rear wall of the front axle bay, driver's side.  The air dryer is opposite, on the front wall.  The emergency tank is beside the wet tank, passenger side, and has the E-filter and the PPV mounted on it.  The accessory tank is under the driver's seat.  The dry tank is (on my bus) behind the rear wall, driver's side, rear luggage bay.  You open a little trap door to get to its drain valve.  The ping tank is on the front wall of the engine side compartment, passenger side, and has the "drain daily" sticker.  For a full tilt, doing your practical test, don't want to fail because the inspector is fuss-budget stickler for the rules, you need to drain all of the tanks all of the way.  What I do around once a year at the beginning of a long trip, is this:  With the compressor having cut-out and the air dryer purged, I open the drain-daily ping tank AKA the exhaust muffler.  If you open it with pressure it can spray right in your face.  Obviously inspect the output like it's your baby's poop, see what your compressor is putting out.  Next, I drain the wet tank all the way down.  I have cables for the wet and emergency tanks routed to the fuel filler space, behind the door.  Look down. see what comes out.  Go in and look at your air gauge - draining the ping tank and the wet tank should not change the gauge reading much, if at all, proving that the check valve between the wet and dry tank is working.  Next drain the accessory tank.  It should drain down, but the air suspension should not change.  The air gauge should drop to 60 psi, proving that the pressure protection valve (PPV) is working.  It closes with a pressure differential of 60 psi between the accessory system and the service system (the dry tank, mostly) in case of a failure retaining air for a last ditch service brake application.  Next drain your dry tank, on my bus the drain valve is behind a trap door on the rear wall of the driver's side luggage bay.  Now your air pressure gauge should read zero.  Final step is drain your emergency tank.  With everything else empty, drain the emergency tank.  It should sound like it has full 120 psi air pressure when you start to drain it, proving that the check valves that isolate the emergency tank from every other system are working.  This is a reasonably complete test of all of the critical check valves and drain valves of the complete circa 1980 air system, at least on an MCI.  There are other things you can do, one variation tests the shuttle valve and one easy one tests the push pull valve.

The push pull valve operates the parking brake.  It is fed by the emergency tank.  What you can do is chock the bus tires front and rear, do this whole blow-down sequence with the parking brake not applied, and when you finally drain the emergency tank the push pull valve should automatically apply via it's internal spring when the emergency tank gets down to around 25 psi.  There is no pressure gauge on the E tank, so I'm happy if it pops up at some point when air is getting noticeably low.  The shuttle valve is connected to the bottom half of the brake application foot pedal valve, which is fed from the E-tank.  It constantly compares application pressure from the service tank and the E-tank, and if the pressure differential exceeds 45 PSI (I think) it will switch and stop using the service tank to apply all of the brakes and start using the E-tank to apply only the rear brakes via the parking brake port on the DD3 chamber.  This is the beauty of the DD3 - it gives you a number of driver-controlled emergency brake applications from a completely separate air supply if the service brake system has a sudden catastrophic failure.  I worked out a way to test the shuttle valve, but I did it a few years ago and I forget how to test it now.  I think it involves a hill, with the engine off and the whole air system drained, which seems a little dicey to me.

TMI, I guess, sorry.  Brian

[Dave5Cs]

Brian being we have the same bus my purge valve got crude in it and did stick open. It would not close. Didn't have an extra so put in some new o rings but they were a little small around and didn't seal it. So after getting fuel it started to leak again to the point of no air at all and locked up brakes. I hooked an air line off my shop compressor through the window and under the dash to the Brake connect and got it off the road safely. I removed the air dyer and caped the signal line and no problems since. I drain often for now and have a AD9 ready to put in when I have some time to block it up. That AD2 we have needs to be serviced about once a year or they give you troubles. there is a check valve also in the top line nut on that AD2 that needs a new ball put in every so often also or it won't seal.
Dave5Cs

[akroyaleagle]

For the benefit of new owners that may not know.

There are many great responses above.

I am unfamiliar with most MCIs, (Have operated the newer J style) but systems on any bus must comply with safety features for DOT so they are usually quite similar.

If an air leak occurs, it MUST be investigated and remedied.

Commercial vehicle systems are robust.

I've had my Eagle more than 21 years. In that time, I've found one potential problem with the air. It was a rub spot on the large DOT air supply hose to the left front brake. I replaced it and wire tied a piece of rubber hose over that area, potential problem averted. I found it during a routine pre-trip walk around. I encourage you to adopt the habit of frequently just looking in each wheel well at all the things you can see to spot potential problems. Closely observe things you may not normally pay attention to. Most have not benefitted from being trained by a professional like those of us that driven or taught at Companies with this equipment.

I have automatic purge valves on the bottom each tank. Several times something gets stuck in the valve and a leak occurs.
It's probably dirt. If you hear a leak check the drain near the sound.

I also have the alcohol injection system on my coach. If I expect to travel when the weather is near freezing, I fill it. That prevents freezing in the system.

The ability to stop these things is a must!

[bevans6]

I was able to duplicate the problem this morning so I can at least troubleshoot it.  Yesterday everything worked perfectly, aired up to 125 psi, purged and sat there for hours with no leakage - today it leaked.  What I've done so far is the full tank drain-down cycle that I described above, and everything operated exactly according to plan.  Drained wet tank, dry tank stayed up.  Drained accessory tank, dry tank went to 60 psi (PPV pressure).  Drained dry tank, pressure dropped to zero (proving, BTW, that the dash gauge reads the dry tank pressure).  Checked emergency tank, it had full pressure with all other tanks at zero so it's isolation check valves are working.  Started engine, ran pressure up to cut-out, which is a tad high at 125 psi, purge was normal but air continued to exhaust from the open purge valve.  The purge valve opens and closes correctly under the control of the governor and does not leak when closed, so I think it is OK.  Air pressure stabilizes at 100 psi on the dash gauge.  The cut-in pressure specification on the D2 governor (which is pretty new) is 105 psi, so a dash reading of 100 suggests to me that the governor told the purge valve to close which terminated the leaking.  This means that a check valve is causing the back-feed to the air dryer, so my next steps will probably be to investigate the check valves on the air dryer and the wet tank.  I can't access my dry tank, it's in a sealed compartment that I don't know how to get into.  If anyone knows, I would love to hear - MC-5C

Brian

[bevans6]

Thanks to all those who said it was the purge valve, because that's what it turned out to be.  The governor send air pressure to the purge valve to open the valve and hold it open until pressure reaches cut in pressure, 100 lbs.  That air pressure goes into the top of the purge valve and pressed down a plunger that opens the bottom of the valve.  The air pressure was leaking past an O-ring into the bottom of the valve, and since the valve was open, out the exhaust port.  As soon as air pressure dropped and the governor cut in, there was no more air pressure to leak so it sat at 100 lbs for a really long time.  Obvious once you take the purge valve apart and see how it really works... 

Cheers, Brian

[windtrader]

The typical MCI of that era has the wet tank on the rear wall of the front axle bay, driver's side.  The air dryer is opposite, on the front wall.  The emergency tank is beside the wet tank, passenger side, and has the E-filter and the PPV mounted on it.  The accessory tank is under the driver's seat.  The dry tank is (on my bus) behind the rear wall, driver's side, rear luggage bay.  You open a little trap door to get to its drain valve.  The ping tank is on the front wall of the engine side compartment, passenger side, and has the "drain daily" sticker.  For a full tilt, doing your practical test, don't want to fail because the inspector is fuss-budget stickler for the rules, you need to drain all of the tanks all of the way.  What I do around once a year at the beginning of a long trip, is this:  With the compressor having cut-out and the air dryer purged, I open the drain-daily ping tank AKA the exhaust muffler.  If you open it with pressure it can spray right in your face.  Obviously inspect the output like it's your baby's poop, see what your compressor is putting out.  Next, I drain the wet tank all the way down.  I have cables for the wet and emergency tanks routed to the fuel filler space, behind the door.  Look down. see what comes out.  Go in and look at your air gauge - draining the ping tank and the wet tank should not change the gauge reading much, if at all, proving that the check valve between the wet and dry tank is working.  Next drain the accessory tank.  It should drain down, but the air suspension should not change.  The air gauge should drop to 60 psi, proving that the pressure protection valve (PPV) is working.  It closes with a pressure differential of 60 psi between the accessory system and the service system (the dry tank, mostly) in case of a failure retaining air for a last ditch service brake application.  Next drain your dry tank, on my bus the drain valve is behind a trap door on the rear wall of the driver's side luggage bay.  Now your air pressure gauge should read zero.  Final step is drain your emergency tank.  With everything else empty, drain the emergency tank.  It should sound like it has full 120 psi air pressure when you start to drain it, proving that the check valves that isolate the emergency tank from every other system are working.  This is a reasonably complete test of all of the critical check valves and drain valves of the complete circa 1980 air system, at least on an MCI.  There are other things you can do, one variation tests the shuttle valve and one easy one tests the push pull valve.

Well, at long last, I finally got all dirty and found the wet tank, after first scouring the MCI manual, air system chapter. It appears on the MC8 there was a change of location of the wet tank. Around 1976, year of my coach, it was moved from the rear between the drive axle to the front. On the wet tank there is a purge valve on the bottom.

I aired up the coach and gave the valve a pull. Wow, what a blast of air, dust and pebbles went all over, including my face. LOL There was a slight amount of oil and nothing else, no crud, no water, etc. This is the first time I checked it since I got the coach which has run about 2500 miles over the past three months. Appears the air system is in pretty good shape

[buswarrior]

windtrader, beware that over the course of mC8 production, FMVSS 121 came into effect, and then shortly, the anti-lock brake bits were rescinded... there were also lingering state specific safety requirements, that FMVSS 121 standardised.

There are no fewer than 8 different air schematics for an MC8 coach.

And then there are the crazy things that previous owners may have done to the coach...

Be sure you start with the correct schematic for your unit number!!!

Happy coaching!
Buswarrior