DD3 Brakes

[moose]

Hello al
to all of you intelligent people
i have a 1970 mci 7 , 2 weeks ago i was in the okanogan valley and my brakes did not release on the passenger side ,i drove for a while ,heated them up finallly got them off ,the garage says the rear seal was gones because i heated the brakes up .
to make a long story short i put new service pods on my DD3 brakes now i just push in the push -pull valve and my brakes are off ready to roll ,before the service job i had to apply the brakes after i pushed in the push pull valve , whay is it different now ,your help is appreciated
trevor

[Lin]

I am not an expert on this.  I have noticed that sometimes that happens.  I believe that DD3 brakes, unlike spring brakes, do not lock as soon as you pull the valve.  I have gotten in the habit of applying the foot brake immediately after pulling the valve.  That seems to make sure they are locked.  I have had it where I pulled the valve, did not apply the foot brake, and have the bus roll even though I thought the DD3's were engaged.  I think if they are really locked, it will take a strong foot brake application to release them.  If you can move without doing that, they may not have been fully engaged.

Experts, please correct me if needed.

[bevans6]

Here is my understanding of it.  There are three ports on the DD3 - the service port for normal use, the emergency port for both emergency and parking brake use, and the locking collar port.  If there is air pressure (above 40 psi by memory) at the locking port, the locking clamp that holds the push rod out is deactivated and the brake cannister is set for normal use.  When the push/pull valve is operated to set the parking brake, two things happen at the same time - air is removed from the locking port, so it can lock the pushrod, and air pressure is applied to the emergency.parking brake port via the 85 psi pressure regulator.  This applies 85 psi to the parking brake diaphragm, which is smaller than the service diaphragm, to apply the parking brake.  If you then release the push/pull valve at this point, the parking brake will release on it's own, because the locking ring has not yet actually engaged the pushrod.  If you apply more pressure via the foot pedal, the push rod will have moved a bit and the locking ring will lock, so just releasing the push/pull valve should not release the parking brake without a foot pedal application.  If the air pressure in the emergency/parking brake tank bleeds off for some reason, below the 85 psi set by the pressure regulator, then the push rod will try to retract and the locking ring will engage, and again you will need to apply greater pressure to get it to release when you take the parking brake off.  The locking ring is a sprague clutch, btw.

So it is possible and normal for the parking brake to be able to be set, and then released with just the  push/pull valve.  If you apply greater pressure than just the normal 85 psi to the parking brake port by "setting" the brake with the foot pedal, then it should require a greater psi brake application to get the parking brake to release.  If the parking brake is on, and air pressure bleeds away for some reason, the locking ring will engage and hold the parking brake set regardless, until released with greater pressure than when it was set.

Hows that?  Did I get it this time, finally?  Disclaimer - MCI, circa 1980.   Earlier ones I think had a slightly different setup.

Brian

[bevans6]

Lin, I think that if you bus can roll if you just pull the valve, something is wrong.  My MCI operators manual specifically does not require a foot pedal application to set the parking brake, and my bus sets it just fine without a foot pedal operation.  I will say that I usually do as you do, and do a pedal application after I pull the valve though, it makes me feel better.

On my bus, if that was happening, I would suspect that the inversion valve wasn't removing the air pressure from the locking port, so the locking ring didn't lock, or the 85 psi pressure regulator was set wrong so the parking brake diaphragm didn't get sufficient pressure to set the brake.  You can test it by pulling the valve while moving at a slow pace, it should bring the bus to a fairly sharp stop just by pulling the valve.  Then, without touching anything else, chock the bus, drain the wet tank, then the dry tank, then the accessory tank, and then finally the emergency/parking brake tank, and see if you can move the bus (helps if you are on a tiny slope, since you can't start the engine since it will just fill up the tanks again).  If you can, then the locking ring did not engage correctly.  If you can't, then the parking brake is locking on properly.  Or you can observe the extension of the pushrod on the brake cannisters, which is a PITA with the wheels on, but probably the correct thing to do.

Another good test is to chock the bus, engine off, and drain all the tanks in the same order as above, and while you are draining the emergency brake tank the push/pull valve should pop up and set the parking brake automatically when you get the parking brake tank down to about 40 psi.

Brian

[luvrbus]

Lin, depends on how they are plumbed Eagles with DD3's if you don't have over 100 lbs of air pressure and hit the brake pedal after pushing the parking brake they won't release and that is on B-12 of a Eagles drivers manual.
fwiw I have changed a few Eagles over to release with just the parking brake valve you just never know how a bus is plumbed Greyhound Eagles would release and apply with just the valve,Trailways would not they would apply but not release

good luck.

[buswarrior]

My guess is that the old diaphragms were leaking air, which allowed the lock rollers to engage fairly quickly.

The new diaphragms have solved that.

Leave it in gear, as described, and confirm it really is parked. What happens after a day of leak down?

Brian's description is sound.

Operator's manuals have always instructed a full service application to release the DD3, whether it was really needed or not is of no consequence to the hired help, "just do it" was the order of the day.

Some time ago, Jim Shepard shared an OEM schematic of how his Eagle's spring brakes had a trick circuit that required a service brake application in a spring brake equipped coach in order to release, mimicking the DD3 procedure. A good defense against misadventure!

If kids/pets are around, when arriving at the campsite, draining the appropriate tanks in order to prevent unintended parking brake release isn't a bad idea.

happy coaching!
buswarrior

[zubzub]

Chock your wheels.  Whether you have DD3/spring brakes or whatever.  I keep a set in the stairwell, makes me feel good.  Plus it's my son's job to put them  on.

[pipes]

A snip from my OLD web page

http://pipesusmc.tripod.com/Kaykay.htm

We lost Nik Nik Today January 10th, 2003 he had a great life. He'll be sorely missed  :-(..  

Nik Nik Turner certified Bus Mutt, he's on his way to the D O L to get his C D L  he  had stepped on the DD3 brake button while we were in a restaurant and as result had his first driving test in Oregon. I had to jump in the rolling bus and slapped my hand on the foot brake just as it rear ended a car and almost shoved everything into the front of the restaurant