DD-3 air brake operation overview (rev 2.0) (quite long)

[bevans6]

Errors or serious omissions please let me know so I can edit.  Otherwise, enjoy...    Comments appreciated, btw.

Brian



DD-3 Air Brake Operation Overview

The intent of this article is to describe, at a reasonable level of detail, how a typical bus air brake system that uses DD-3 brake chambers operates.  It's based on my bus, a 1980 MCI MC-5C, so it should be relevant to most MCI's from that era.  It's important to note that other bus systems may have slightly different implementations of certain elements of the braking system, so the expectation is that owners will learn and understand how their system works.  It's also important to note that this isn't an entry level course on Air Brake 101 or Air Systems 101, so I'm not going to cover what a relay valve is or a quick release valve does.  Those are stories for another day.

DD-3 Air Brake Chambers

There are three broad categories of air brake chambers.  There are plain brake chambers, which have one diaphragm only, and are used on front wheels and tag axle wheels in most buses.  There are spring brake chambers, which have a service brake chamber with a diaphragm and a separate spring chamber that holds a large emergency/parking brake spring.  These are very commonly used on trucks, trailers and many buses.  The spring is used to lock the wheel in the absence of air pressure creating both the emergency stop if air pressure fails, and the parking brake.  There are probably other quite old systems that you will have to figure out on your own.  The Bendix DD-3 chamber is the subject of this article for two reasons – first, it's very commonly used in many buses between the 1960's and the 1990's, and second, it's old enough that it's no longer taught in most classes, either on-line or in schools, and it's not covered in DOT or driver's license examinations.  This means that learning about them is not only hard, it's haphazard and full of inconsistencies.  My hope is that this article, along with the peer review of fellow posters, will make it all a little easier.

The DD-3 chamber was developed by Bendix with the idea of creating a two section brake chamber with a service brake section, a separate parking/emergency brake section, and a push rod lock parking brake.  They are easily identified by having three air ports where spring brake chamber have two ports and plain brake chambers have one port.  The most common DD-3 chamber has a 30 square inch main diaphragm operated by the middle port.  The rear port operates the parking/emergency brake diaphragm, which is about 15% smaller than the service brake diaphragm so that the same amount of air pressure will develop about 15% less application pressure.  The locking port controls a sprag clutch, a group of rollers that bear on the push rod is such a way as to not allow it to retract back into the chamber unless there is air pressure applied to the locking port.  If there is no air pressure at the locking port the push rod cannot retract.  This is what lets the DD-3 chamber act as a parking brake.

The Dual Air Brake System

This is based on a 1978 MCI brake system.  That's important because air brake systems after around 1977 had to have a dual tank system so that if one system failed losing air pressure the other system could still operate and bring the vehicle to a safe and controlled stop.  Some earlier DD-3 brake system designs may be different than described here, so you have to get your air system schematic out and follow along to trace out what I am going to describe.   Many spring brake systems accomplished this with two service tanks, one nominally for the rear brakes and one for the front brakes, with a two way check valve that allowed one tank to substitute for the other if one side developed a catastrophic leak.  A serious flaw in this idea is that if one tank loses all air pressure and the other tank is brought into action through the check valve, the leak still exists and the air in the second tank still gets lost.  This is where the spring brake systems have a third fully mechanical fail-safe – the springs start to mechanically apply the brakes if the air pressure drops below 65 psi and they will bring the vehicle to a halt – but not under driver's control.

DD-3 dual brakes have two air tanks that are completely separate – the main service tank, also called the dry tank, and the emergency/parking brake tank.  Air first comes into the main service tank and from there flows to the emergency/parking tank through a check  valve (and some other stuff which we can ignore for this article).  Once the air is inside the emergency/parking brake tank it can only go to either the parking brake system or the emergency brake system.  These two systems share many of the same components, but serve different functions.   The DD-3 emergency brake system is designed to allow the driver to make "several" fully controlled brake applications should the main service section fail completely and lose all air pressure.  After pressure in the emergency brake air system falls to a set point the parking brake will apply automatically.

NOTE:  A key difference between a spring brake system and a DD-3 brake system is that a DD-3 brake system does NOT have a fully mechanical fail-safe.  The brakes in a DD-3 system CANNOT be applied without air pressure.

The E-9 Treadle Valve

The service brakes and the emergency brakes are operated by the treadle valve, also called the E-9 dual brake valve or the foot control valve.  There are different ones in use depending on the age and design of the bus system – my 1980 MCI has an E-9 valve.  The E-9 valve is a dual brake valve and has two separate control valves, both operated by a common foot pedal.  In normal operation, the primary section of the valve is fed with air from the main service tank and it sends controlled air directly to both the front brake chambers and to the relay valve that operates the rear brakes.  In this way the primary section of the E-9 treadle valve uses the primary air supply from the service brake tank to operate both the front and rear brakes in normal driving.  The secondary section of the E-9 treadle valve is fed from the emergency/parking brake tank.  It controls that air feed in the exact same manner as the primary section of the E-9 treadle valve does but it sends its controlled output to the shuttle valve instead of to the brakes.  The shuttle valve is the primary control valve that allows the DD-3 brake system to act as a dual braking emergency brake system.

The Shuttle Valve

The shuttle valve is the control valve that decides if the emergency braking system will be  activated and controlled by the secondary (emergency) side of the E-9 treadle valve.  It has two inputs, from the primary and secondary outputs of the E-9 treadle valve, and one output.  It compares the pressure presented by each input, and if the difference in air pressure is greater than 30 to 40 psi, it closes and allows the air pressure from the secondary output of the E-9 treadle valve to pass through a two way check valve and directly operate the emergency/parking brake section of the rear DD-3 brake chambers.  It does not pass any air to the front brakes, and only operates the rear brakes.  It has access to the full air pressure retained inside the emergency/parking brake tank, but – recalling that the emergency/parking brake diaphragm of the DD-3 is about 15% smaller than the main service chamber diaphragm – it can only develop about 85% of the maximum braking force of the normal service brake system, and of course only on the rear axle.

The Parking Brake Push Pull Valve

The parking brake system and the emergency brakes system use many of the same components but are controlled separately.  The parking brake is controlled by the Push Pull Valve, located beside the driver.  The push pull valve is fed with air from the emergency/parking brake tank.  It has a spring controlled auto-application function that will function when the air pressure in the parking brake tank falls below a set point, usually around 35 psi.  This is the final stage of the emergency brake system – once air in the emergency/parking brake tank falls to a dangerously low level, the parking brake system applies with the residual air.

When the push pull valve is operated air is fed from the emergency/parking brake tank to the inversion valve through an air pressure regulator set at 85 psi.  On my MCI this air pressure regulator is located on the passenger side front bulkhead of the rear axle bay.  The purpose of the parking brake air pressure regulator is to limit the amount of pressure that the parking brakes can be set with to ensure that they can be released with a normal brake application when the push pull valve is set to parking brake off.

The Inversion Valve

The inversion valve is the central control point of the parking brake system.  It is located on top of the differential section of the rear axle.  It is fed with an 85 psi air supply from the parking brake air regulator.  It is controlled by push pull valve.  It has two outputs - the first output is to the locking port on the DD-3 brake chamber and the second output is to the emergency/parking brake port on the DD-3 brake chamber.  The reason it is called an "inversion valve" is that the two outputs are always opposite, or inverted, to each other.  If there is pressure at one output there is zero pressure at the other output.

In normal service brake operational mode, the parking brake is off and the locking port on the DD-3 is unlocked.  This is accomplished by the push pull valve being released, which tells the inversion valve to release air pressure to the emergency/parking brake port on the DD-3 brake chamber and to send 85 psi air pressure to the locking port.  That pressure releases the sprag clutch that locks the push rod.  When the service brakes are applied, the DD-3 brake chamber push rod is pushed out a tiny amount, which lets the sprag clutch rollers retract and the push rod is freed up to move in and out normally.  Note that air pressure is always present at the locking port whenever the bus brake system is in normal service mode, with the parking brake off.

In parking brake operational mode, the push pull valve is pulled up into the "ON" position, which tells the inversion valve to change to its other mode, removing air pressure from the locking port and applying 85 psi air from the parking brake air pressure regulator to the emergency/parking brake port on the DD-3 brake chamber.  This pushes the DD-3's push rod out, applying the brake with a regulated force, and locks the push-rod out so that the force is held.

To release the parking brake, the push pull valve is pushed down, sending a signal to the inversion valve to apply air pressure to the locking port and release air pressure from the parking brake port on the DD-3 brake chamber.  Then, a service brake application is made which pushes the push rod further out, the sprag clutch lock releases the push rod and normal service brake operation is again possible.

Summary

The Bendix DD-3 service brake chambers have two distinct and separate sections – the main service brake section and the parking/emergency brake section.  They also have a push rod lock.

In normal service brake operation, the front and rear main service brakes on the bus are operated by the primary section of the E-9 dual brake valve using air from the service, or dry, tank.  The push rod lock is released during service brake operation.

In emergency brake operation, the shuttle valve detects a difference in air application pressure from the E-9 dual brake valve, and the rear brake emergency brakes only are operated by the secondary section of the E-9 dual brake valve, via the shuttle valve, using air supplied by the parking/emergency brake tank.  The push rod lock is released during emergency brake operation.  As air pressure falls to a pre-set level the parking brake will be automatically applied using residual air pressure.

In parking brake operation, the push pull valve is operated sending a signal to the inversion valve, which operates to send regulated air pressure from the parking/emergency brake tank to the parking brake section of the DD-3 brake chamber while simultaneously releasing air from the locking port.  This pushes the push rod out to apply the rear brakes and locks the push rod to hold the brakes on until released.

[lostagain]

Great write up. I just noticed how you say the push/pull valve is pulled up to release the brakes, and pushed to apply them. Should be the other way around. Everything I have driven, bus or truck, is pull to apply, and push to release.

JC


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[bevans6]

Fixed, thank you.  I always remember that wrong...

Brian

[Debo]

Thanks for the great write-up Brian. That took a sizable effort, and I appreciate the clear, concise, and accurate information. This should be made into a sticky note to permanently reside on this site. Thanks again.

[Kitt]

Thank you for taking the time to write that up.

[bevans6]

It's kind of funny, that was a straight brain dump onto the page, it took me about an hour and a half to write because that's how long it takes me to type and proof-read a three page Word document.  No research, just type it out.  But it's taken four years to get to the point where I understand the system well enough to do that, and I am absolutely still learning about it.  It took about two years for me to figure out the inversion valve, then a light went on - "its a remote control A-B switch".  I felt like a real dummy.

Brian

[Oonrahnjay]

It took about two years for me to figure out the inversion valve, then a light went on - "its a remote control A-B switch". 

     That one sentence is worth reading this board for a month!!!   Thanks, Brian   

[bevans6]

I am currently smoking over the shuttle valve.  It operates at a pressure differential of 40 psi...  What will it do if the main air pressure is at say 65 psi, the low air warning has just come on, the emergency tank has full pressure at 120 psi and you do a light 20 psi brake application?  Will the application activate the main service brakes, the emergency brake system, or both at once?

Brian

[Emcemv]

Brian, thank you very much for this write up, being a newbie, I am still struggling with understanding the brake system.  I've learned how to adjust it properly but have not yet understood it's operation fully, your document really helps, I'm going to print it out and keep in my manual.

Bruce