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Author Topic: Parking brake tank pressure - need advice!  (Read 3099 times)
bevans6
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« on: July 12, 2009, 12:36:34 PM »

I'm reading the pressure at the emergency fill valve of my MCI MC-5C, and am reading zero with the bus up to full 120 PSI pressure.  My schematic shows that valve directly connected to the parking brake tank.  I can apply and remove the parking brake.  When I drain the parking brake tank bethind the front wheels it has pressure.  I removed the schraeder valve from the fill port, there is no pressure.  Any ideas?

The reason I checked this was to check out the pressure protection valve.  I wanted to be sure it was passing full pressure after the supply tank got up to 65 psi.  My schematic flow shows air coming from the supply (dry) tank through the filter to the PPV, then to both the accessory tank, and to the parking brake tank though a check valve.  The emergency fill valve is shown directly connected to the parking brake tank, and also to the dual brake valve, the push pull valve and the inversion valve through an 85 psi pressure regulator.  That's a lot of important stuff to not be getting the right air pressure!

Thanks, Brian
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« Reply #1 on: July 12, 2009, 01:12:39 PM »

You mentioned a check valve. Is that positioned such that pressure readings aren't possible at that port? Or perhaps there's another check valve not on your diagram.

Another possibility is line blockage...has that valve accepted shop air previously? You should be able to tell if it's flowing.
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« Reply #2 on: July 12, 2009, 02:00:30 PM »

I've just about decided that I probably leapt to a conclusion that the schraeder fill valve was also the one way check valve, but it may make sense that there is a second one way check valve in the line prior to the fill valve.  I'll investigate that tomorrow.  I will also investigate a way to put a gauge on the accessory tank.   I also think I may have a faulty governor or something wrong with the compressor.  The governor seems to either lag a lot after it's below cut-in pressure, or the compressor isn't holding air pressure in the wet tank and has to fill it before it can fill the dry tank.  When I took pressure readings after an hour run, there was no air pressure at the ping tank, but pressure at the dry tank.  I'm going to pop out and take readings now, it's an hour and a half since I last aired up the bus.

Brian
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bevans6
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« Reply #3 on: July 12, 2009, 02:34:13 PM »

Well i have pressure in the wet tank, the parking brake tank and no pressure at the ping tank.  Here is what I think is happening, please help me understand if I have this right!  the compressor builds pressure up to cut-out.  The governor cuts out and sends a signal to the air dryer to purge.  The purge empties the air from the line between the ping tank and the air dryer, so there is no pressure between the compressor head and the air dryer.  The one-way check valve in the air dryer maintains air pressure in the wet tank.

In other words, my system is working as it should be.  I think.  If I turned off the bus, stopping the compressor, while the compressor was building air (say 100 psi on the gauge) then I should see the pressure being maintained in the ping tank, since it would not have been purged off.  I will try that tomorrow.

I am determined to figure out exactly how this system works!

Brian
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« Reply #4 on: July 12, 2009, 06:28:18 PM »

The ping tank (I call it a muffler) has no pressure when the comp is not running.

There is a check valve after it to keep air from  bleeding back through the comp.

Even when it is running the pressure there jumps all over the scale.
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Gary '79 5C
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« Reply #5 on: July 12, 2009, 06:29:48 PM »

Bevans,

Do you have a schematic ? Not to sound like a smart a$$, but I should be able to dig on out from my maint manuals and email same.
I need to do so as I have a small leak in my tank in the driver side rear baggage compartment. Me thinks it is the remainder of the factory leveling valves which might be still pressurized.
This one system I have not conquered in knowledge, but need to do so.
I can air up, guage at 120, but I do not hear the governor "dump". I was inside the hanger with fast idle for 25-30 minutes.

Email me if you wish, if you need the schematic, garypasternak@aol.com

Take care,
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bevans6
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« Reply #6 on: July 13, 2009, 03:57:01 AM »

Thank you, I have the factory maintenance and operators manual.  I actually have a original, OEM as it were, operators manual for the 5C.  That's where they call the discharge muffler a "ping tank".  It does a real good job of accumulating moisture and oil out of the compressor, I drain it every time I drive the bus.  My problem, if I have one, is that the schematic is incomplete.  It doesn't show the air dryer, where pressure is read, the location of the switches, etc.  That's fine, I'm getting to learn this.

The tank in the rear drivers side bay is the dry tank for the brakes only, it does not serve the suspension.  Air is fed through a check valve, filter, pressure protection valve into the accessory tank then to the suspension.  I would treat a known leak in the dry tank as fairly important.

Also, you can tell that the governor "dumped" if the air doesn't go past 120 PSI.  it would continue to rise and trigger the safety valve at around 150 psi if the governor wasn't working.  What you hear is the air dryer purging, which it does when signalled by the governor.  If it isn't purging then maybe the signal line is bad or the air dryer is bad.
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« Reply #7 on: July 13, 2009, 04:16:28 AM »

Gary, Brian is right about the tank in the rear bagage bay. For rear brake only, feeds air to a relay valve then to the rear brakes. When the pedal is applied air is sent to the relay valve opening it. Air from this tank then applies the rear brakes. Not a spot for a leak..  Tom Y
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« Reply #8 on: July 13, 2009, 04:41:19 AM »

Air from the dry tank in the rear bay supplies all of the air for all of the brakes (on a 5C anyway), and supplies all the other tanks through various protection and check valves.  It's directly connected to the R-8 relay valve which activates the rear brakes, but is also connected directly to the E-9 dual brake valve up front, worked by the pedal, from there it feeds the front service brakes through a quick release valve, and also feeds the signal line from to activate the R-8 relay valve.  Unlike modern truck (or maybe coach) systems which  have separate service tanks for front and rear brake systems, the DD3 system has all the brakes served from single tank for normal operation and a separate "parking brake" tank for emergency use that can feed air into the main system through the shuttle valve.

I still have no idea what the inversion valve does.  Haven't got that far yet!  Soon!

We should have an air system tutorial thread.  I think there are a lot of people who could benefit from a deeper understanding of how things work and the air system is arguably the most important system on the bus!

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
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« Reply #9 on: July 13, 2009, 04:56:04 AM »

We should have an air system tutorial thread.  I think there are a lot of people who could benefit from a deeper understanding of how things work and the air system is arguably the most important system on the bus!

I think you just started one! Wink

Go here, you might get your questions answered. http://www.bendix.com/en-us/service/library/Pages/Home.aspx
http://www.bendix.com/en-us/service/library/Pages/Bulletins.aspx


Good Luck!

Paul
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« Reply #10 on: July 13, 2009, 05:32:28 AM »

Brian, My bad about the rear tank. You are right. I had to go back to the book, should have went there first. My take on the inversion valve, for what its worth. Looks to me that it applies the park brake when the presure drops off.   Tom Y
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« Reply #11 on: July 14, 2009, 04:58:19 PM »

Brian,

We should have an air system tutorial thread.  I think there are a lot of people who could benefit from a deeper understanding of how things work and the air system is arguably the most important system on the bus!
You are very much correct, I need to correct this and like soon. I plan to get closer to this over the weekend.

I will keep in touch. I have had some confusion as to the schematic and the tank locations/ purpose.

I appreciate all the 35'ers assistance.

Gary
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« Reply #12 on: July 15, 2009, 07:54:40 AM »

I have three questions illustrated in the attached diagram.
I know these are basis questions, but I do not wish to make a poor assumption.

1. Is the Ping tank just off the air compressor, like within 2' ? A small 2" dia, 8" long tube, with threaded bottom with a blow down valve ? Wet tank as described in diagram.

2. Is my dry tank in the 5C is located in driverside rear baggage comp ??

3. R-8 Relay Valve is located on rear center bulkhead of rear baggage compartment.

Does the R-8 relay open/close with brake application? Does this also balance between the two rear brakes?
I need to look for the air leaks and I believe they start out from this Dry Tank. Sounds like dinosaurs snoring after shutdown.
Thanks,

Gary
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bevans6
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« Reply #13 on: July 15, 2009, 08:28:50 AM »

1. Is the Ping tank just off the air compressor, like within 2' ? A small 2" dia, 8" long tube, with threaded bottom with a blow down valve ? Wet tank as described in diagram.  

Yes, that is the ping tank, as they call it in the operators manual.  Drain the ball valve on it's bottom daily, mine has a schrader valve to let me fill there from an external air supply.

2. Is my dry tank in the 5C is located in driverside rear baggage comp ??

Yes, that is the dry tank.

3. R-8 Relay Valve is located on rear center bulkhead of rear baggage compartment.

I believe so.  the R-8 valve has two feeds from the dry tank, a signal line from the dual brake application valve (the "pedal valve" up front) and has an air linethat go, probably through a quick release valve, to the rear foundation brakes (the main part of the DD3's).

Does the R-8 relay open/close with brake application? Does this also balance between the two rear brakes?

The R-8 valve is what applies supply tank air pressure to the rear brakes.  It gets the air directly from the dry tank.  The signal line from the E7 foot valve tells it to send air to the brake cans.  it sends the air to a quick release valve, which sends the air to the brake cans through identical length air hoses.  The quick release valve is what splits the air to the cans and what vents the air to atmosphere when the brakes are released.  Sending the air to the rear brake cans through the quick release valve and then through identical length hoses to the cans is what balances the brakes side to side.  The front brakes are engaged through a quick release valve and identical length hoses too.  The main thing this does is make sure the time lag to apply the brakes is the same side to side,  The reason the R8 valve is directly connected to the dry tank is to ensure an adequate supply of air, and to reduce the time lag for the air pressure to find it's way from the front of the bus to the back of the bus.  The signal line is quite small diameter compared to a main hose line, so the air signal travels quickly from the E7 foot valve to the R-8 valve.
  
I need to look for the air leaks and I believe they start out from this Dry Tank. Sounds like dinosaurs snoring after shutdown.

There is always pressure between the dry tank and the R8 valve.  There is no air pressure after the R8 valve unless the brakes are applied.  So check the R8 valve and those connections.  Probably just replace the R8 valve if it hasn't been replaced lately, they are routine maintenance items.  If you hear noise towards the front of the bus, look at the air dryer, sometimes the purge valve doesn't close.  


I hope this helps you out.  It is my understanding of the R8 valve, anyway.  did this from memory, my 5C manual is at home.

Cheers, Brian

This site has some good stuff, this in particular is a guide to troubleshooting air brake systems.  The R8 relay valve is number 14 in this diagram.  Keep in mind the differences between a DD3 system and a spring brake system with primary/secondary tank system.  http://www.todaystrucking.com/images/ArMerTruckTroubletp9974.pdf
« Last Edit: July 15, 2009, 08:39:12 AM by bevans6 » Logged

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« Reply #14 on: July 15, 2009, 09:10:51 AM »

Many thanks Brian,

Just discussing this provides me with a clearer head to get the understanding I needed. My two holding tanks are just in front of the rear bulkhead, so it may time for me to remove the hot water tank, and lose 30 lbs to gain access & look between tanks. I have not looked for this valve before, but will tonight. I see also that the dry tank has a check in the feed line from the wet(ping) tank. I doubt that is leaking as I always have pressure at the blowdown.

Are there any tanks upfront under the steering & frt axle? and would that be the park brake tank?

Thanks again,

Gary
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« Reply #15 on: July 15, 2009, 10:14:46 AM »

My current understanding is that there are five tanks on the bus - in order of air flow - ping tank (which is not really a tank, just a little tube), wet tank, dry tank, then parking brake tank and accessory tank through the same line.  Air flows from the compressor to the ping tank, which takes off the worst of the water and oil.  From there it flows to the air dryer.  From there it flows through a check valve (one way valve) to the wet tank.  The check valve retains air in the wet tank when the air dryer purges.  When the air dryer purges it drains all the air in the line to the compressor including any air in the ping tank.  From the wet tank the air flows to the dry tank, through a check valve.  The check valve retains air inside the dry tank.  From the dry tank, air flows to the R8 valve (two lines, no check valves) to the E7 foot valve (one line no check valve) and to the parking brake and accessory tanks through a check valve and then an air filter.  From the air filter it goes to a pressure protection valve, then splits and goes to the accessory tank and to the parking brake tank (may be a check valve on the parking brake tank, retaining air in the PBT).  From the accessory tank it feeds the suspension and all the gadgets (like doors, clutch assist, various air cylinders).  The parking brake tank feeds various parking brake valves and the E7 foot valve with a shuttle valve as an emergency brake air supply.

Again, I don't have my schematic with me, but this will be close.  The R8 valve will be in plain sight inside the rear axle space.  I think it is on the front wall of the rear axle bay, not inside the luggage bay.  Just listen for where you hear leaks to start.  Remember which lines are supposed to have air in them all the time, and which lines are suppose to have air in them only when the brakes are applied.  Get air pressure up, turn the bus off and do an extended brake pressure test - do a 100PSI application for 10 or 15 minutes and see what is leaking, how much pressure you lose.  See if you can hear anything different than what you hear with the brakes not applied.  See if you hear anything different if you turn the bus off right after the air dryer purges compared to if you turn it off while the compressor is cut-in and feeding the bus.  All of this will let you know what part of the system is leaking.

Brian
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« Reply #16 on: July 15, 2009, 10:29:23 AM »

Brian, My R-8 valve has 1 line to the dry tank. A line to each brake chamber and the line to the E-9 or E-10 valve. My book shows either a 9 or 10 and I am unable to find out which. MCI was unable to tell also. Do you have a E-7 ?  Tom Y
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« Reply #17 on: July 15, 2009, 10:40:57 AM »

I will check my manual when I get home tonight.

Brian
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« Reply #18 on: July 15, 2009, 05:19:03 PM »

Well I looked at the rear bulkhead inside the bagg comp. and I could not believe that Sam Caylor blocked access to the R-8 valve, and he did not. I do remember seeing valves inside the rear wheel well and will get to them this weekend. I did see a airline hose with a plug in the end coiled up in the rear bagg. I will look to see where this feeds from and remove. I do not know if the PO had a air assisted toilet, as this is in the neighborhood? I try to clean up any and all "deadends" I find air or wiring.

Another very basic question, I do not know where my air dryer is located. I see the Ping Tank, but the line exiting IIRC travels to the center of the bus and then forward.

I did located the tank in the driverside rear baggage comp. Is this the Dry or Wet ? ? If the dry, as I believe then where is the wet tank, after the Ping ? I do not remember seeing a wet tank between the ping & dry tanks.

Kinda on the run tonight, but look to research and get this correct this weekend.

Thanks All
Gary
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« Reply #19 on: July 15, 2009, 06:23:54 PM »

The air dryer is on the front  wall of the front axle compartment, kind of on the drivers side.  the dry tank is behind the rear wall of the back off the drivers side luggage bay, which you seem to have found.  My schematic shows the foot valve as an E9 Dual  Brake Valve.  Schematic also shows two lines from the dry tank to the R8 valve.

Do you not have the 5C maintenance manual?  You really need to get one if you don't!

Brian
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« Reply #20 on: July 16, 2009, 04:24:29 PM »

Brian,
I do have the manual, however from all my questions I do not fault you for asking. The schematic is not exactly straight forward (at least to me) in regards to location. The description within the manual is not verbose either.

I do greatly appreciate the dialog and direction, as I have a mucho better grasp on things. I do remember the blowdown remenants on the floor under the steering area and this must be the dryer.

Actually, I have just one more question, Where does the air system blow down at the top pressure ie 120+ psi Huh

Gary
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« Reply #21 on: July 16, 2009, 06:39:37 PM »

There are a few steps to the blow down process, also known as air dryer purging.  Which gives you a big clue as to where the air burst comes from, actually!  First things first - the air compressor is always spinning and producing air.  When the air pressure gets below the governor cut in pressure - 95 psi say - the governor can tell this because the air pressure that the governor senses via a signal line from the dry tank gets low enough that it can no longer overpower a spring in the governor, the spring presses a shuttle valve to one side, and allows the air that the compressor is producing to be sent to the wet tank.  On the way to the wet tank it first goes through the air dryer, which on our buses is located at the front drivers side of the front axle compartment, just behind the drivers seat area. It's a can about 6" in diameter and 18" or so tall.  So the air compressor does it's thing and the air pressure in the signal line to the governor from the dry tank gets high enough to press the governor spring again, and it does so when the air pressure reaches cut out pressure - 120 PSI.  By law, the range between cut in and cut out cannot be more than 25 psi.  So the governor cuts out, and in doing so pressurizes another signal line that goes between it and the air dryer (one way to tell if you have an air dryer is there are always two  small diameter signal lines connected to the governor.  If you don't have an air dryer, there is only one signal line).  That signal line tells the air dryer to purge, and it does so, releasing all the air inside the air dryer can and the line to the compressor, blowing any trapped moisture out.  A check valve operates, isolating the wet tank from the air dryer so all the air in the wet tank isn't lost.  

The schematic makes a lot more sense when you take an air-brake course at a driver training school or a local trade college.  The governor circuit is not shown in the manual at all.  I took a 12 hour course a few weeks ago to get my Z-endorsement.  That's the only reason I know this stuff - I learned it very recently at school!  And not being a teenager any longer (for several decades), I paid attention and finished first in my class.  I think I pissed off all the guys who were there to get school bus licences and truck licenses because they had been laid off their regular jobs.  Because I finshed first I got to do the practical exam first (on a tractor trailer rig) and go home first.  I felt quite bad for them, being out of work around here is not a laughing matter, we're a Canadian steel town and that's not a good place to be unemployed just now.

sorry for the digression...  Hope this helps.  Oh - when you look under the front axle bay, there are two air tanks on the rear wall.  the drivers side one is the parking brake tank, and the curb side one is the wet tank.  There are drain valves two inches up from the bottom on the front of the tanks with little rings (some nice person may have put lanyards on the rings).  You reach in with a push rod and push or pull the ring, that blows the air out.  Always drain the wet tank first, all the way to empty, then drain the other tanks.  sometimes the drain valve hangs up when you release it when the tank is empty, and after you start the bus to air it up it hisses at you, just smack it a bit with your stick that you used to push it with.

Brian
« Last Edit: July 16, 2009, 06:49:15 PM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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« Reply #22 on: July 16, 2009, 08:06:08 PM »

This is real interesting. My 5-A has the wet tank mounted in the rear axle area attached to the rear bulkhead of the baggage bay. The Brake tank and auxiliary tank are on a bulkhead in the front axle compartment and there is another much smaller tank located beneath the drivers area electrical junction box. No air dryer. I added lanyard operated valves to all tanks because they had petcocks on them before. It all seems to work fine but I wish i had an air dryer I think.

And i agree the schematic in the MCI manual gives no indication of actual tank location; its a logical diagram only.
Fred
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« Reply #23 on: July 17, 2009, 05:52:50 AM »

Fred, you may have some of the tanks named wrong, but then again maybe not, you know your bus better than anyone else.  Does the 5A have the DD3 system with a parking brake tank?  You don't mention a parking brake tank, which on a DD3 system is always separate from the dry tank.  Anyway, on my 5C the auxiliary tank is under the driver's side junction box area.  The wet tank, with or without an air dryer, is almost always a long distance from the compressor so that the air has it's best chance to cool and condense water, that's why it's in the front axle bay.  It's equally important for the dry tank (or the main supply tank for the braking system) to be very near to the rear brake relay valve.  It needs to be very near since it supplies the air for the rear brakes directly via that relay valve and the distance must be short to minimise application delay lag time.  That is exactly the same reason that the parking brake tank is at the front of the bus, near the foot valve.  the distance between the parking brake tank (which is also the emergency brake air supply) to the shuttle valve and the foot valve must be minimised to reduce application lag time.

BTW, I've been wondering how to tell where the dash gauge reads from.  People have said it reads from the auxiliary tank, and I felt that it could not, since by law it has to read the pressure in the main service supply tank (as is the current proper name for the dry tank). Well, since there is a pressure protection valve between the dry tank and the auxiliary tank, the aux tank gets no air when pumping up from scratch until the pressure in the dry tank exceeds 65 psi.  if the gauge read from the aux tank, it would read zero for three or four minutes, then jump to 65 psi  quite quickly.  It does the opposite of that, it does exactly what you would expect if it read directly from the dry tank.  It starts to show a gradual gain in pressure almost instantly from when the engine starts and the compressor starts filling.  It goes up to about 65 psi and then stalls - as it gains access to the parking brake and auxiliary tanks and brings them up to 65 psi.  Then it starts to rise again, at a slower pace, as it is now filling the entire system volume with air.  That observation is one test of the pressure protection valve.  If your system doesn't do that, then test the pressure protection valve.  Likewise by draining the tanks in a particular order you can test the operation of all of your check valves.

The DOT test for compressor recovery operation is to gain full operating pressure, and then, with the engine running, apply the brakes to pump the pressure down to 85 psi and to record the time it takes to build pressure to 100 psi.  That time must be less than 2 minutes, and is typically between 30 andd 40 seconds.  As important is to track the time for your bus - if it starts to increase, that's a sign of a fault in the process of failing.

As I expected, the mere exercise of thinking through these scenario's is increasing my understanding of the air systems in the bus tremendously.  I truly hope the real experts are monitoring this and will jump in to say yea or nay  to our observations here.

Brian
« Last Edit: July 17, 2009, 06:00:10 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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buswarrior
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'75 MC8 8V71 HT740




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« Reply #24 on: July 18, 2009, 07:48:37 PM »

Hello from the bushes....

Depending on the vintage of MCI, the location of various tanks changes.

Earlier, the wet tank was in the rear, later, it is up front.

Through the 1970's, there are some 8 different schematics for the MC8, depending on year, unit number, and original jurisdiction of purchase/operation, as some states had unique "safety requirements".

There are significant plumbing changes between the various MC5 variants as well.

As always, we must be sure of our own unit's particular peculiarities and whether they have been faithfully maintained and/or properly retrofitted to some other conventional configuration by those who owned it prior to our adventure.

And mine might not be like yours!

Otherwise, carry on, the post is pretty good!

happy coaching!
buswarrior

 
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Frozen North, Greater Toronto Area
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