MC-5C air brake 101 primer

[bevans6]

A thread on the Vintage Bus group Facebook group led me to think a primer - a MCI air brake 101 - might be handy.  At the risk of annoying people I thought that if it was here as well, it might be findable for future reference - I know I find stuff here when I do google searches.  This is based on a MC-5C 1978 air brake system, which I am familiar with as my bus is a 1980 MC-5C, so is effective unit 7401 per the schematic. Earlier models might be a little different, later models get more complicated, and I don't cover those changes. This is a pretty long post, and I just literally typed it as fast as I can type - it's a stream of conciousness dump of all I know about how MC-5C air brakes work.  The schematic of the 1978 vintage MC-5C air brake system is at the bottom

Air system basics: The compressor is located on the back of the engine, and is gear driven at engine speed. It is controlled by the Air Governor, which is typically bolted to the compressor but in some conversions may be remotely mounted. The compressor supplies air to the air system, and the governor controls the air pressure to between 95 psi and 120 psi, the normal air system operating range.

The air compressor sends the air to a Discharge Muffler, located on the forward wall of the engine bay, just inside the side door. The Discharge Muffler slows the air down, traps moisture and oil, and has a Drain Daily sign on a valve, which you should open and drain daily during routine operation.

From the Discharge muffler air is sent forward to the Air Dryer, located on the forward wall of the front axle bay. The air dryer dries the air, and purges the moisture when the air governor tells it to, which is when the air governor turns the compressor off (unloads the compressor) at 120 psi. Air flows from the air dryer through a one way check valve to the Wet Tank. The wet tank is part of the Service Brake, or Foundation Brake system (the main air brake system) and is designed to further trap moisture. It also has a 150 psi safety valve. The wet tank is located on the rear wall of the front axle bay, on the driver's side (by the rear axle on early models, I think, particularly ones with no air dryer)

Air flows from the wet tank to the Dry Tank through a one way check valve. The dry tank, AKA the Service Tank, is the main source of air for the service brake system on the bus. On this vintage of bus (1978 - 1980) the air pressure gauge on the dash shows the pressure in the dry tank. The dry tank also is the supply of air for all of the other air sytems on the bus, and air flows through an E-Filter, to a Pressure Protection Valve, and at that point the service brake system stops and the rest of the bus air systems begin. More on them later.

The pressure protection valve is critical. It (and the E-filter) are located on the Emergency Brake Tank, located on the rear wall of the front axle bay, passenger side. The pressure protection valve is closed when the air pressure in the service tank is less than 65 psi, and it opens when air pressure is over 65 psi, so allowing the rest of the air system to fill. If a failure occurs in the suspension system, for example, the PPV will close when air pressure has fallen to 65 psi and retain some operating air inside the service brake system.

For main service brake operation, air is supplied from the dry tank to the E-9 Dual Brake valve, located under the driver's seat and operated by the brake pedal. The E-9 valve has two separate systems, one being used for normal service brake operation and the second being used for emergency brake operation (more later). In normal operation, air is supplied to the E-9 valve supply port one from the dry tank, and the output from port one goes to both the front brakes directly via a quick release valve, and through a small control line to the R-8 Relay Valve, located on the forward wall of the rear axle bay. The R-8 valve is fed with air directly from the dry tank, and is controlled by the E-9 valve, and it operates the rear brake chambers. In normal service brake operation, the brakes are controlled by the brake pedal, which controls the E-9 valve, which sends air directly to the front brakes and which controls the rear brakes through the R-8 valve.

Parking Brake Operation: The MC-5C, and most MCI buses from the beginning up to the mid 1990's, use the Bendix DD-3 Safety Actuator (google it) as the rear brake chamber. It's main service brake chamber is a 30 square inch chamber, and in normal operation, is supplied with controlled air via the R-8 valve. It has an second port that supplies air to a separate parking and emergency brake chamber, and there is a third port that controls a push-rod locking device. The parking brake operation is controlled by the Inversion Valve (the most confusing valve known to me, if not all of mankind). The Inversion valve is located on the differentional housing of the rear axle. The inversion valve is the heart of the DD3 parking brake system, and is itself controlled by the Push Pull Valve, located beside the driver's seat. In normal operation, the PPV is pushed down (releasing the parking brakes). Air is supplied to the PPV by the Emergency Brake Tank, and when the PPV is down (normal operation) air is supplied from the PPV to the Inversion Valve control port, which then sends air pressure to the locking port of the DD3 to unlock the push rod. The inversion valve itself is supplied with air from the Emergency Brake Tank, through an 85 psi pressure limiter which is located on the forward wall of the rear axle bay, toward the passenger side. More on the 85 psi limiter later.

So in normal operation, the push pull valve is down, air is supplied to the inversion valve control port, which unlocks the DD3 pushrod lock, and the E-9 dual brake valve circuit one causes air to go to the DD3 service brake chamber by controlling the R-8 valve, and the E-9 valve channel one sends air to the front brake chambers directly via a quick release valve

Parking Brake Operation: The DD3 brake chamber has a separate Parking and Emergency Brake chamber, with a separate port. Unlike spring brakes, the DD3 system must have air pressure to apply the parking and emergency brakes, and this air pressure is supplied by the Emergency Brake Tank, which we discussed above. The Ebrake tank is separated from all other air systems on the bus by a one-way check valve - air goes in and only goes out to the parking and emergency brake system. The Emergency Fill port is connected to the Ebrake tank only, and therefore can only supply air to the parking and emergency brake systems.

When the parking brake is applied, the push pull valve is lifted. This removes air pressure from the inversion valve control port, which causes it to "invert" and remove air from the DD3 locking port, causing the push rod to lock, and applies air to the DD3 emergency/parking brake port, causing the parking brake to apply. As it applies, the push rod is pushed out and locked in the applied position, so if air pressure drops for any reason the parking brake will remain fully applied. The air supplied to the parking brake port is limited to 85 psi by the pressure limiter, and the parking brake diaghrapm is 24 square inches (80% of the size of the service brake chamber) with the result that the maximum push rod pressure that can be applied by the parking brake is approximately 60% of that of a full pressure service brake application. This means that a full service brake application can always move the push rod out farther than it was with the parking brake application to release the parking brake. So now you know why you do that. And why you don't do a full service brake appliction just after you apply the parking brake, because you will stick the push rod out too far to release and it will be bad. To release the parking brake, you push the PPV in to normal, air is supplied to the inversion valve control port, which sends air to the locking port of the DD3 to unlock it, and takes air away from the parking brake port on the DD3, and then you do a full service brake application to release the push rod of the DD3 from the sprag clutch rollers hidden inside the DD3.

Almost done, and the best part. It took me three years to figure this part out all the way. Emergency Brake Operation. The E-9 dual brake valve has, as it's name implies, two separate systems, called them One and Two. One operates the service brakes as described above. Two operates the emergency brakes in a very cunning fashion. We talked about the Ebrake tank being separated from the rest of the bus air system by a one way check valve. In normal operation, the Ebrake tank fills up to 120 psi and sits there, waiting for a parking brake appliction - or to be needed for the emergency brakes. It supplies air to the second side of the E-9 dual brake valve through supply port two, and send the identical modulated air supply out delivery port two to a Shuttle Valve. The shuttle valve compares the pressure of air supplied from the E-9 brake valve delivery ports one and two to see how far apart they are. Remember that the Air Governor is controlling the air compressor so that it comes on at 95 psi and comes off at 120 psi? That is a maximum difference of 25 psi. If the shuttle valve sense a difference in the modulated air pressure from the two ports on the E-9 valve of more than 35 to 40 PSI, it "shuttles". When it activates it starts send the air signal from port two of the E-9 valve directly to the emergency brake port on the DD3 chambers, so that the rear brakes only are now controlled by the E-9 foot valve with air suppled by the Emergency Brake Tank. Even if air pressure in the service brake system falls to zero, the Emergency brake tank holds enough air for several controlled brake applications so you can drive to a position of safety and stop the bus. In this instance, you remind yourself to NOT PUMP THE BRAKES, and be as smoothly controlling of the pedal as possible to release the smallest possible amount of air from the system. The push pull valve is supplied with air from the emergency brake tank, and when the air pressure in that tank (not the service brake tank) falls below it's set point (35 psi in my bus) it will self-activate, pop up under spring pressure, and active the parking brakes with the remaining air pressure left in the emergency brake tank. There is a two way check valve near (or mounted on) the inversion valve so that the emergency/parking brake port on the DD3 can be supplied with air from either of the inversion valve or the shuttle valve.

That is all I got on MC-5C circa 1978 - 1980 brakes. But - a couple of points about the air system that some might not know. The dash pressure gauge reads from the dry tank. You can watch it rise quickly from zero to 65 psi, and then sit there for a while as the rest of the air system starts to fill. After several minutes all is up to 65 psi and the pressure starts to rise again, slowly now as the whole air system is pressurized all at once. Then, the air suspension is slowly filling at taking air as it comes to pressure and levels the bus. It's important to know that the air governor that controls the air compressor is fed from the auxiliary air system - on the other side of that pressure protection valve we talked about. Until the air system gets up past 65 psi and the auxiliarly air system is filled, the air governor does not know what the air pressure is. If your gauge - reading the dry tank - says you have lots of air pressure, but your goveronor won't switch the air compressor off, your suspension isn't filling and nothing else works, maybe it's the PPV... The air supply to the skinner valve that operates the "engine stop" lever, and everything else back there in the engine compartment (shutters, air cylinders that tension belts, the low air switch, the generator cut-out switch, everything back there) is fed by the same line that feeds the air governor, so they are all part of the auxiliary air system and won't work until the PPV (that bad boy again) lets air into the rest of the air system as the bus pressures up. So learn how to turn the engine off with the no-fuel lever.

[GnarlyBus]

Excellent Post!

Bendix has manuals and service data for most of the components in the MCI Air Brake System. These are available free online. They are really good with excellent diagrams and helped me understand the system.

They also have some good YouTube videos that show how the system works.

[buswarrior]

Lovely job Brian!

Can't emphasize enough the importance of knowing which vintage air system regulations that a particular bus has been built to.

Spring brake knowledge just gets you into trouble with a DD3 system.

Bus brakes are NOT all the same in very significant ways from WWII to the present.

Be safe and read lots.

Happy coaching!
Buswarrior

[Jim Blackwood]

From what I've learned so far about the '96 102DL3 it seems to be very similar but they added fancy features to the suspension. Makes for a good start at least. Mine has the dual gage so I'm still figuring out that sequencing business. Hopefully by the time I start driving it I'll understand it a little better.

Jim

[bevans6]

Jim, your air brake system has a few similarities to the one I described, but also significant differences.  I think this excellent note from MCI describes your system;  http://www.mcicoach.com/fyiFromMci/maintMatters/0215.htm

Note two separate air tanks feed the dual foot valve - primary for rear brakes and the secondary for front brakes.  Note a second inversion valve that impacts the emergency braking function.  Still has a separate parking brake tank, but it no longer has an emergency brake function - that is now supplied by the secondary air tank.  They added an anti-compounding function to the Push Pull Valve (parking brake valve) so that if the foot brake is applied when the parking brake is on, the parking brake gets released.  The early MCI systems did not have this feature, and it's to prevent over-pressuring the parking brake.

[luvrbus]

Every bus should have a dual air system since 1975 ,I have spring brakes on my DL now, I am not a huge fan of the old DD-3 brakes never have been buses are  the only place you find the DD-3.Eagle went from DD-3's to spring brakes in the 80's ,buses are getting so heavy now they are coming with parking brakes on the tag axle that project is next on my list spring brakes on the tag

[chessie4905]

Yeah, I went through that with my H8H649. I rebuilt or replaced all the brake items, including the tanks. I have manuals for both single and dual systems. Many more components with dual systems, and tanks have different functions with dual systems. You must be careful not to misconnect  components and lines and carefully follow Correct diagrams. The GM inversion valves in this system are specific and one is no longer available even through Luke. I had to buy currently available valve, which took 3 months to get the first one and 3 weeks for second one I realized I needed after waiting for the first. I then swapped the guts and shims from original because they had different pressure ratings.

[bevans6]

My view is that the only advantage of DD-3 is the ability to drive the bus to a stop if that catastrophic failure happens, while spring brakes are just going to stop you right now if the air pressure falls below 65 lbs.  Spring brakes win out in just about every other way - simple, safe, automatic, no air - brakes are on.  Cheaper to install and maintain, and can be caged to tow.  If a DD3 locks on and you can't get air to it to unlock the pushrod, or it won't release, you have to pull the wheels and physically de-mount the can to get it to release.  At some point that ability to drive to a stop must have been considered pretty important.

[buswarrior]

This stuff all got invented back when there were no brakes.

Lots of designs fell by the wayside.

When spring brakes first came out, the truckers were up in arms about the chance to burn the trailer to the ground with a dragging spring.

DD3 was air operated both sides.

If it wasn't for MCI's fascination with them, they too would have vanished decades earlier.

Just a different way to park a bus, with some emergency stopping capability long before anyone regulated it.

Happy coaching!
Buswarrior

[luvrbus]

Yep and even at the over inflated price at truck stops you can walk out with a spring brake chamber for under a 100 bucks.lol don't try that with a DD-3 chamber not going to happen you set and wait for one to be shipped at a fortune   

[chessie4905]

Bevans6, that is not quite right. GMC's have levers at the slack arm to rod connection that allow applied chamber to be released with a wack of a hammer. It removes enough pressure from brakes to allow movement to get off the road. You reset with a large screwdriver or medium prybar and re-bend locking tabs. I don't know if it was standard or an option, but my coach has them. They will retrofit to any DD3's.

[luvrbus]

Bevans6, that is not quite right. GMC's have levers at the slack arm to rod connection that allow applied chamber to be released with a wack of a hammer. It removes enough pressure from brakes to allow movement to get off the road. You reset with a large screwdriver or medium prybar and re-bend locking tabs. I don't know if it was standard or an option, but my coach has them. They will retrofit to any DD3's.

My MCI 8 had the release lever too I have seen those on a lot of Eagles from the 70's with DD-3's

[bevans6]

That's a good trick!  My bus has a clevis pin, that's all.  I wonder if it was a retrofit or an option?  Is that standard on GM's?

[chessie4905]

Easy to change over. I don't know if it was standard. I think it was an option.

[Jim Blackwood]

Thanks for the link Brian, I'll spend some time going over it. May have some questions if you don't mind too much.

Jim