DD3 parking brake issues

[RJ]

Brad -

What you're experiencing is not uncommon for newbies to DD-3 brakes, so welcome to our world!

Craig outlined the proper release procedure above, but he left out a small, but important detail:

When applying the service brakes after pushing in the knob, push the pedal to the floor and HOLD it for 3-5 seconds, then release. At this point the bus should roll.  If not, repeat. If they still don't release, you've got other issues that must be resolved.

Also, when applying the parking brake, only cover the service brake enough to keep the coach from rolling, which will allow the parking brake to function properly.

FWIW & HTH...

[buswarrior]

You said the coach moved backwards?

Then the drums and shoes are not rusted seized.

If the parking control valve popped trying to release, I'd be wondering if there is stuck or blocked valving in the air system, leaving some portions with less air than the gauges indicate.

Just keep at it, air up, attempt release, see if you can get to 120 lbs on gauge, air up, attempt release.

Give the air some time to seep through whatever it is that isn't moving correctly.

That air system needs a good a proper kick in the @$# to get working again.

On the brake chamber end of things, previous owners are notorious for not giving the locking rollers in the chamber a LITTLE grease now and again.

With enough agitation, you should get it to release, however, the reliability of it all requires some serious preventive maintenance to be caught up.

The inversion valve controls everything, mounted on a bracket attached the differential, all the air lines go to and from it, tap it with some authority while the parking control valve by the driver is in the release position. Please block the wheels so you don't get run over, if fooling around under neath, suspension blocking also comes along shortly so you don't get crushed into the ground like a bug, read up about that elsewhere on here.

There are aftermarket DD3 available from.... (help me out, REM, RPM, MNR, DOT, IRS?).... much cheaper than from MCI.

My strategy is to avoid crawling and tools as much as possible, keep hammering away from the driver's seat for a LONG time...

happy coaching!
buswarrior

[bevans6]

aftermarket new or rebuilt DD3 cannisters - http://www.rebuildersenterprises.com/

DD3 parking brake 101...

On my bus, an earlier generation MCI (circa 1980) than the posters, the push-pull valve is sent air from the emergency brake tank.  It sends air to the inversion valve, to tell the inversion valve to put the DD3's into drive mode or park mode.  In drive mode, air is sent to the locking port to hold it open, in park mode air is removed from the locking port to allow it to lock the pushrod, and air is sent to the parking brake port to push out the push rod.  When the inversion valve is told to switch back to drive mode it send air to the locking port again, letting it unlock, air is removed from the park port so the push rod is released, and the big push of air from the brake pedal pushes the push rod out a bit so the push rod sprag clutch rollers can actually unlock.  So that is how that works.

The PP valve and the inversion valve work on air sourced from the emergency/parking brake tank, not the normal brake air tank.  The emergency tank is separated from the rest of the bus air system by a one-way check valve, so it airs up to 120 psi and just sits there until some air is used for parking or emergency use.  Air delivered to the inversion valve and used by it for it's purposes goes from the emergency tank through an 85 psi pressure regulator and then into the inversion valve.  That limits the pressure that can be delivered to the parking brake port and makes sure that it doesn't apply too hard, making it difficult for the brake to release.  That's why you shouldn't "set" the parking brake with an extra application of the foot pedal after you push the PP valve to apply the parking brake.

Finally - there is a spring inside the push pull valve that will automatically operate it, and apply the parking brake, when the air pressure in the emergency tank falls below a set air pressure, around 35 PSI.  At that point the PP valve switches and the remaining 35 psi of air sets the parking brake.  That is the fail-safe.

There is a shuttle valve that is part of the emergency braking system as well.  It is connected to the treadle foot valve, and monitors the air pressure delivered from the foot valve to the brakes.  It is sent air by both sides of the treadle valve (which is a two channel system), one side is fed from the normal dry tank that is the service brake air supply tank, and the other side is fed from the emergency tank (it does a lot for one tank).  If there is a fault with the service brake tank and air pressure falls so that the shuttle valve senses a difference of more than around 40 PSI in the two channels (I forget the actual pressure difference and I don't have the manual to look it up), the shuttle valve decides the main service brakes have failed and "shuttles" so that the air from the emergency tank is fed directly to the parking brake port on the DD3 (through a two way check valve) so that you get several solid brake applications on the rear brakes only, through the emergency tank and the parking brakes.  Once that air is exhausted down to 35 psi, the PP valve operates and the parking brakes apply using the residual air.  There is no mechanical way to apply the parking brake, no big spring that sets them like spring brakes have, but the advantage of DD3 is that you get to control an emergency stop when your air supply fails.

There were several updates to this basic idea after my bus and before DD3's were abandoned, but this is a kind of sketchy overview of whats involved.

Brian

[lostagain]

Brian, again an excellent description of the DD3 parking brakes function. I must point out though, that the push/pull valve is pulled to apply, and pushed to release the parking brakes. You might want to edit your post to correct that.

JC

[Raymond smith]

Just a thought on the front start switch. On my C3 i have to have the step light on or the front start won't work. Safety or theft deterrent?  who knows. HTH. Raymond

[RJ]

Just a thought on the front start switch. On my C3 i have to have the step light on or the front start won't work. Safety or theft deterrent?  who knows. HTH. Raymond

Raymond -

Is your coach an ex-Greyhound? The step light/starter interlock has been used by Hound for years as a theft deterrent...

FWIW & HTH...

[Raymond smith]

Yes it an old hound. Good to know. Thanks for the info. Raymond

[Jim Eh.]

Also air is released from the "park" chamber when the dash valve is pulled out to set the park brakes (thus the whoosh sound)

Air is supplied (to overcome the parking/emergency spring) to the spring chamber when you push your dash valve in to release the parking brake.

[bigred]

My Prevost refused to move once like yours .Even though front and rear ap was 120 pounds .Called my bus Guru and he went through a spiel that as slow as I type would take me to Monday to tell all of it ,but the gist was to keep hitting the brake pedal to get the pressure down low enough to get the compressor working and when it aired back up ,end of problem!!!

[bevans6]

Brian, again an excellent description of the DD3 parking brakes function. I must point out though, that the push/pull valve is pulled to apply, and pushed to release the parking brakes. You might want to edit your post to correct that.

JC

Push, pull... english is my second language...  I talk fluent "forget" most of the time...  I always get that wrong.

I do think that if you are going to drive a bus with DD3 brakes, let alone pretend to maintain or work on one, knowledge of the system to this level of detail in mandatory.

Brian

[Gary Hatt - Publisher BCM]

Here is a blurb I just extracted from the MCI website.  Maybe this will also shed a bit of light on this subject.

Park Brakes - DD3 Brake Chambers
MCI coaches, with the exception of the Renaissance coach, all use DD3 brake chambers on the drive axle for the Park Brakes. When the park brakes are applied, air pressure is routed to the rear of the chamber that applies the brakes. At the front of the chamber are rolling "locks" which wedge themselves between the shaft and the body of the chamber. If the air were to leak off of the park brake diaphragm, these locks will hold the brakes in the applied position. To release these locks, you follow the steps listed below:

Start the coach and allow the air pressure to build to 120 pounds.
Release the park brakes by pressing down the park brake push/pull valve in the driver's area.

Firmly apply the Service Brakes. This causes the brake chambers to apply further than the park brakes had applied, and this releases the rolling "locks" from the shafts.

Two problems that have been encountered are listed below. Also listed will be the troubleshooting procedure for fixing it.

Park brakes will not release.

Usually having too much air pressure applying the park brakes causes this. In the air supply line for the park brakes is a pressure regulator which is set to 85 pounds. This is the amount of air pressure used to apply the park brakes, not the 120-pound system pressure. If this regulator is set too high, or the regulator has been removed, the brakes will be applied too firmly to be released by the service brake application. Setting this regulator too low will affect the stopping/holding capabilities of the park brake. 85# is the magic number to remember.

Air is leaking out of the R-14 relay valve located above the drive axle (On ABS equipped coaches, air leaks out of the modulator valve.)

In the DD3 brake chamber, the park brake diaphragm separates the park brake section and the service brake section. If this diaphragm develops a hole in it, air will leak into the service brake section. When the service brakes are not applied, this section is vented to the exhaust port on the relay valve (With ABS, the exhaust port on the modulator valve.) Any air entering the service brake chamber will be exhausted out immediately thus causing the illusion of a faulty relay valve (With ABS, the modulator valve.)

To troubleshoot this situation, remove the service brake airline from one of the DD3 brake chambers. (This is the middle hose on the chamber) If air is leaking out of the service brake port, that is the chamber with the leaking park brake diaphragm. If air is still leaking out of the relay valve and the hose in your hand, then it is the chamber on the opposite side of the axle (On coaches with ABS, air leaking out of the RH modulator valve would mean the leaking park brake diaphragm will be in the RH DD3 brake chamber, LH modulator valve would point to a leak in the LH side.)

[GiddyInn]

I would like to thank everyone for all their helpful and friendly advice. We finally got Giddy Inn home!! Apparently everyone underestimated my ignorance on charter busses. I learned a lot today. After I crawled under the buss a couple of times, greased the roller locks, tapped on the valves with a hammer, and nothing working yet, called the mechanic to see if he had time to come out and check a few things. While sitting in the drivers seat waiting to hear back from him, was playing with all of the dash switches to see if they did anything. Then I saw what I thought was an ashtray lid behind the shifter, thought that was a weird place for an ashtray. Opened it up and found a black knob, looked closer to find out it said "Parking Brake"  We had been trying the yellow knob on the dash near the door controls that said parking brake on it. apparently that knob is just for the storage compartment locks. Long story short, bus is home, ran great, we are more educated then we were, and have our first bus story to tell! Again thanks for everything!!

[GiddyInn]

Hopefully a picture loads.

[B_K]

I would like to thank everyone for all their helpful and friendly advice. We finally got Giddy Inn home!! Apparently everyone underestimated my ignorance on charter busses. I learned a lot today. After I crawled under the buss a couple of times, greased the roller locks, tapped on the valves with a hammer, and nothing working yet, called the mechanic to see if he had time to come out and check a few things. While sitting in the drivers seat waiting to hear back from him, was playing with all of the dash switches to see if they did anything. Then I saw what I thought was an ashtray lid behind the shifter, thought that was a weird place for an ashtray. Opened it up and found a black knob, looked closer to find out it said "Parking Brake"  We had been trying the yellow knob on the dash near the door controls that said parking brake on it. apparently that knob is just for the storage compartment locks. Long story short, bus is home, ran great, we are more educated then we were, and have our first bus story to tell! Again thanks for everything!!

OK Giddy first off we didn't underestimate your ignorance of charter BUSES, but rather gave you the benefit of the doubt due to your introduction about your schoolie experiences.

Now that said I suggested you needed to be edumacated on MCI's and was hoping a busnut near you would chime in and offer up some assistance in this area. 

Glad you have it going and sounds like someone replaced the bay door lock knob with one from a tractor trailer.
But hey look at it this way if a would be thief knows as little or less about buses than you did it is an excellent security device! 

Glad you got it home and now the real edumacation can now begin!
Feel free to ask away, if we don't know we'll make something up!
  BK 

[gumpy]

 

Ok, that's funny! Someone put a brake knob on the bay doors! Cheap security, and evidently it works!

Well, now you have a heck of a story to tell regarding your first experience with your new bus. Don't feel bad or ignorant. We've all made mistakes like this. Someday when you post a question saying the bus is shutting down 3 minutes after you start it, I'll tell you about how I learned the hard way about the rear stop/run switch!

Glad you figured it out.