Parking brake tank pressure - need advice!

[bevans6]

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

[Tom Y]

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

[bevans6]

I will check my manual when I get home tonight.

Brian

[Gary '79 5C]

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

[bevans6]

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

[Gary '79 5C]

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

Gary

[bevans6]

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

[Fredward]

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

[bevans6]

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

[buswarrior]

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