Air system question

[Lin]

When I start the bus the air pressure gauge stays put for a while even though I know it is gathering air.  Soon, the needle will start to climb at a reasonable pace.  The air bags inflate at around 90 psi, although sometimes it waits till the gauge is higher.  When I shut down the engine, the needle will drop to zero withing ten minutes, although there is actually still some air in it.  When aired up, it always plays between around 105 and 120 psi.  Is the gauge within normal limits, or should I be replacing it?  Also, I bought a cheap compressor for airing up at campgrounds.  It only puts out about 100 psi, but I thought that would be close enough to get started.  I connected it to the bus and let it run for about 25 minutes.  Although the compressor was pegged at 100 psi, the bus gauge didn't move off zero.

[Tim Strommen]

That sounds to me more like there may be an issue with where the gauge is hooked up, or how many air leaks you have...

If the gauge is mechanical - it should show pressure whether the engine is running or not (so long as there is pressure in the tanks).  However if the gauge is electric (and has a "sender"), it may not get a signal once the engine is stopped (ignition is off).

Staying between 105 and 120 is about right - this is your governor unloading the engine-attached compressor when it hits 120 and allowing devices (brakes/leveling valves) to draw air pressure until it gets to 105 then kicks the compressor back in...

You probably have an accessory tank that the air bags are attached to.  There is typically a protection valve which prevents air from being sent to that tank until the rest of the air system (which operates the air-brakes) gets over about 85psi.  This way if your accessories have bad leaks - the vehicle is still safely drivable.

I've drawn up an overly simple diagram of how the air tanks are typically hooked up in a air-brake vehicle.  Note that there are between three and four tanks on different variants of vehicles (in some vehicles, the wet tank is also used as the accessory tank).  The two tanks you are most concerned with are the primary and secondary brake tanks (front and rear brakes respectively).  These are the ones that bring your coach to a safe/controlled stop and need pressure to do so.  The gauge(s) should be hooked directly to those tanks. If they are, and you are still seeing all of your air lost within 10 minutes from a full tank at shut-down, you will need to go over your air lines and valves to look for leaks.

Keep us posted...

-Tim

[Lin]

Thanks for the response.  I don't think I have a rear gauge.  At least have never seen one.  I have some around though and will install it near the compressor.  There is some sort of cylinder about the size of a regular compressor air dryer in the right engine compartment.  This device has a quick disconnect on the bottom of it that can be used for assorted things, but I also use it as a point to fill the system.  I can easily install a gauge there.  It will give a somewhat better picture of what is going on.  According to the schematic, it looks like the front gauge is attached to the line from the wet tank.  Of course, that may have changed over the years.  Is there any reason that there would be different pressures in different parts of the system or any reason that part of the system would fill before another?

[tekebird]

by frt and rear gauge he means front and rear tank gauges.  some gauges have 2 needles.

[RJ]

Lin -

On your MC-5A, based on it's 1965 vintage, you do not have a dual air system.  If you did, there would be two needles within the air pressure gauge, one green and one white. 

I think you'll find, if you trace back the plumbing from the dashboard gauge, that it is plumbed into the accessory tank in the compartment below the driver.  In this system, the accessory tank is the last one in line from the compressor to receive air, which is why it takes awhile for the needle to start moving after you fire up the coach. 

Tim's also right about the protection valve - it won't let the accessory tank start filling until the service brake tanks have about 85 psi in them.

If you pay attention, you'll also notice that the belt-driven alternator tensioner (if you still have it),  and the cooling blower belt tensioner also don't kick in and tighten up the belts until the system hits around 85 psi.  That's because they're plumbed off the accessory tank, also.

When the air gauge bleeds off within 10 minutes of shutting down, there's probably a leak somewhere in something that's plumbed off the accessory tank, be it wipers, suspension, tensioners, etc.  It's amazing how fast a leaking cooling blower belt tensioner will bleed air out of the system!!

If you decide you want to install additional gauges, be careful where you tap into the system to do so, and be sure to label them appropriately.

FWIW & HTH. . .

[gus]

Lin,

I find that when airing my 4104 the gage comes up to max pressure about the same as yours and will drop off quickly, also like yours, if I don't run it very long.

I decided this is caused by the air bags not fully inflating in a short time even though the gage shows full pressure. I think the air bags full inflate only after everything else has reached full pressure, probably because the bag inlets are pretty small compared to the rest of the system.

[Lin]

RJ
  What you say about the gauge probably being tied into the accessory tank makes sense.  That would then explain its slow start and quick descent. There is a small leak in the wiper line that would bleed off a small tank.  There may be others, but that's the only one I'm sure of.   Is there any problem with putting a gauge between the compressor and the wet tank?  I could even just slip it into the quick release there and remove it when I needed air for something.

[Barn Owl]

My bus airs up about the same way. It takes time before the gauge starts to move. Recently I started to loose air pressure rather quickly (way longer than 10 min though) and I thought with that much air loss surly I could hear it but I couldn't. I found my relay valve was the problem so I am going to change it out. I still have the original Bendix R5 which is now obsolete. Bendix told me to go with the R12. Anyone already done that?

[Sojourner]

Lin.....http://diesel.btc.ctc.edu/Brakes/Bendix%20AirBrakeHandbk4_2002.pdf

Barn Owl...R-12 come with vertical air line holes and horizontal air line holes. Get the one is best suited for your application.

FWIW

Sojourn for Christ, Jerry

[buswarrior]

Hello Lin.

You have an MCI, with DD3 brakes. You are plumbed differently than Tim's schematic.

I expect that you have 4 tanks, a wet tank, a parking tank, a service tank and an accessory tank.

The air gauge is, as previously suggested, plumbed into the accessory circuits.

Your parking and service tanks, the ones that are of highest priority for brake functioning, as well as trouble shooting leaks, are blind to you.

Replacing the tank drains with air fittings lets you plug air gauges into each for trouble shooting purposes.

If you have air on the gauge, you have air everywhere else. Unless you test the pressure protection valve, you cannot assume there is air anywhere else when the gauge is down, or that the brakes are protected from a suspension blow out, which is of concern for every busnut.

If the air is leaking down as fast as you describe, you really want to make that stop. Track down the leaks and eliminate them.

A practical goal for a busnut might be that the coach should be able to be shut down for a fuel stop and the leaks are small enough that it is able to be started and driven away immediately after paying for the fuel.

happy coaching!
buswarrior

[Lin]

Yes, I will check the air pressure in the main system.  I believe that it is the accessory tank that is draining.  In the type of situation you mention-- fueling and starting up-- although the gauge may drop to zero, it is back to 120 psi within 60 seconds.  But this is really flying blind without having a gauge on the main system.  Thanks

[Tim Strommen]

Hi Lin,

It looks like you've gotten a bunch of good model-specific info from others.  Based on the feedback of the gauge being plumbed to the accessory tank, I think you are getting some "bad info" to your dash.  For the sake of safety, you may want to consider plumbing that gauge to the primary brake tank - since this is the most critical to you while under-way.  Attaching a gauge to the accessory tank where is can be viewed from the passenger side of the bus (curb-side, not the road side of the bus) should give you a good troubleshooting gauge if you notice a problem with your accessories.  A "low-air" idiot light for your accessory tank is probably all you need at the dashboard, while a sounder/light and gauge is best for the brake tank(s) - so that it calls attention to a critical situation before you are left with no service brakes (i.e., the spring brakes apply due to loss of air pressure).


The check-valves I had drawn in the schematic above are designed to allow air to go only one way through them - and they will have a very low "cracking pressure" (the pressure differential at which it begins to allow air to pass).  There is typically a small ball-bearing in a cage which gets pushed out of the way by incoming air - but gets pressed against an o-ring seal blocking the passage, if there is more air trying to go back towards the compressor.  These valves are usually attached directly to the air tanks to eliminate any possibility of a feeder hose bursting and causing the rapid loss of brake control pressure.  They have an arrow stamped or forged into the body indicating which way the air flows freely.

The protection-valve has a diaphram and a reaction spring which keeps the valve shut until there is enough air presure to over-come the force of the spring against the diaphram.  Once there is enough pressure (standard protection valve has an 85lb spring), the diaphram moves, unblocking the air passage and allowing air into your accessory tank.  These valves may or may-not include a built-in check valve as well.  They should be attached to the wet-tank directly so that if the accessory tank or the hose from the wet-tank to the accessory tank bursts or developes a large leak - the rest of the air system would be protected to a level of at least 85psi, at the expense of your accessory opperation.

The above diagram shows the two gauges being tapped directly to the air tanks for the primary and secondary air systems.  It sounds like you don't have a front and rear air tank - but rather "service" and "park", separate from the accessory tank.  In my diagram, the wet-tank is NOT directly attached to anything other than the next down-stream tanks.  This is a remnant of the legacy air systems, where a place to catch oil and condensed moisture needs to be provided for accumulation after an air-dryer, or non-air-dryer system's compressor.  This tank is important to drain every once in a while, since it should illustrate how much moisture or oil is in your system (and thus the condition of your air-dryer, if equiped).

Depending on how your rig is plumbed and valved - the primary and secondary air supply will appear at the brake treadle, or parking selector.  Thus your gauges can be plumbed locally at the dashboard.  I'm of the opinion that this is fine for both the gauges and the low-air warning switch (which should turn on a buzzer and warning light below 85psi on the service tanks) - since if you don't have good supply pressure at the brake controls, it won't matter what's in the tanks .  Also the fewer new long hose runs in your air system the better (don't add things that weren't there before... or change lengths).


I don't really see any value in placing a gauge on the compressor line full time.


Finally, yes - tekebird hit the nail on the head, some times there are two separate gauges, and other times there is a single gauge with two needles with different colors in the same dial, indicating the air pressure for a primary and secondary air supply.

-Tim

[bobsw]

Just a reminder block the bus when fooling around under the bus looking for air leeks.

[Lin]

Thanks for all the input here.  Things now make sense.  I will have to wait on the re-plumbing until I have more time, it gets cooler, and I can set up a safe blocking system.  The property is unpaved and is really just packed sand.  It's doable but will require some preparation.  Since I had an old compressor gauge around, I did install it in the back.  The bonus to doing that was to discover that the nipple holding the quick disconnect in place was stripped and apparently just staying there out of habit.  So now, through a mixing of some old parts (I just love using old parts) and some new ones, I have added the gauge and a schrader valve to the set up.  I used my compressor to air up the bus to see what would happen.  The system gauge was at 100 psi before the dash gauge began to move.  My compressor cut out at 110 psi on both gauges.  It took about 45 minutes for the dash gauge to be back to zero.  I had tightened one of the offending wiper lines, but it really needs new ferrals.  In that time the rear gauge dropped to 85 psi.  That was about 4 hours ago, and it's at about 80psi now.  I guess that good news is that the main system is holding well, so it's only the accessory tank that needs attention.  Thanks again.

[buswarrior]

Hello Lin.

My opinion:

MCI plumbed that coach the way they did for good reasons.

The various driver's licence training materials are quite insufficient to be making design and re-plumbing decisions. Especially when the information is based on a modern dual circuit spring brake system, and yours is a legacy single circuit DD3 system.

Get the air schematics for your coach, and understand why MCI has plumbed it that way.

Then test the coach to see if it is performing up to spec.

Unknown to many, an old DD3 system offers most of the same protections as the modern dual circuit system, since the service and park tanks are isolated from one another and act on the brake chambers through separate plumbing.

If there is insufficient air showing on an MCI gauge of that vintage, what is in the other tanks is irrelevant, it should not be driven until you know why, and have the knowledge as to the degree of danger to yourself and other road users to continue.

happy coaching!
buswarrior