DD3 Operation

[Lin]

My bus, as a few others, has DD3 brakes.  I know that the correct way to release them is to give it a good, full pressure application of the service brake after releasing the park brake.  I notice that sometimes the DD3's seem to let go as soon as the the valve is pushed in before a serious application of the service brake.  So my question deals with how these brakes function.  Are they strictly on/off, so if the bus rolls even without the service brake application, they are completely released?  Or can they partially release thereby allowing the bus to roll but still providing some resistance?

[luvrbus]

Lin, not a MCI guy but on a Eagle if you apply the foot brake with pressure and set the brake it takes the same amount again to release it set it with no foot pressure and when the air pressure reaches a few pounds over that they release without pedal pressure may not work that way on a MCI.as they may have a quick release valve


good luck

[bevans6]

deleted since it was wrong

[luvrbus]

Never owned a Eagle with DD3 that worked that way I have seen a few Eagles that worked that way,but Bendix said you could not plumb a spring brake to have a constant air supply when released but I do it I don't trust the cans to hold enough air helped others here do their that way also it's all in the plumbing I showed that to my Bendix instructor and he is still trying to figure it out. 




good luck

[buswarrior]

Hold the phone...

As noted, according to spec, the DD3 is applied by some 85 lbs of air pressure from the isolated parking tank.
(In practical terms, they are applied by whatever pressure the regulator applies to them, from the isolated parking tank, all of which is blind to the driver, no gauges involved here. hmmm, I wonder how old that regulator might be...?)

And the air is concurrently released from the locking port, so the rollers are pushed against the pushrod by their ring and spring.

And they just sit there. There is no binding of the rollers until there is a lessening of the pressure against the pushrod, by way of one or one of two issues:

1) the driver had a heavy service brake application applied, stronger than the parking portion's spec 85 lbs, when activating the parking control. In removing the foot application, the pushrod relaxes somewhat, and the rollers are inclined to lock. Once locked, they need a service brake application to get them to free up so the ring may be pushed back by the pressure applied to the locking port when the driver pushes in the parking control.

2) If the driver's service brake application is less than the parking application's spec 85 lbs, or no service application at all, the rollers just rest against the pushrod, no locking takes place until the pressure in the parking tank drops to below the application pressure, then the pushrod relaxes a little, locking the rollers, and the service brake application is needed to release them.

So, if the system is up at 120 lbs whe parking, the parking tank has to leak down below the 85 lbs before the pushrod will relax. And, as mentioned, there is no relationship between the dasg gauge and the parking tank, except when you see the gauge climbing in the normal operating range, when you may assume that the parking tank is at an equivalent pressure, otherwise, the gauge wouldn't be rising.

In conclusion, it is quite normal for the driver to be able to apply the DD3 parking brake, and then be able to get them to release only by manipulating the parking control in most of the DD3 equipped coaches in the hands of busnuts.

The condition of the pushrod and the rollers, rust, grooves, or otherwise less than smooth conditions, the ring and spring that push the rollers back, and the all important grease zerk on the DD3 that lubes these things are all variables, as well as the actual air pressure being applied to the chamber.

Now, best practice by all DD3 drivers is to make a heavy service brake application on release to ensure that the rollers have unlocked. You don't want to burn up a set of brake linings by way of "theory"

happy coaching!
buswarrior