Parking brake does not release all the time

[robertglines1]

I'll add another variable to above post of old hardened grease. Many things can cause either a binding lack of grease or old  harden grease  or as Dave pointed out excessive wear=cam angles etc.  Simple things first.  Good luck! 

[chessie4905]

   Maybe you should consider having the DD3's dismantled, cleaned, and lubed, especially at the rollers and reassembled. They aren't that difficult to service if you regularly work on coach components. Just get the manual and follow it . 

[uturn]

Hi Dave,
 The rollers in the can look ok. I had both chambers rebuilt and tried yet another one I had to be sure. The parking valve is new. All the valves relating to the bus brakes are new. Someone wrote on here that I should change the regulator on the air tank in the back. I don't see a regulator around the tank. It goes directly to the relay. Thanks for the reply. David.

[eagle19952]

I don't see a regulator.....maybe that's your problem....

Clifford said that your application pressure was supposed to be regulated to 85 psi.
maybe you should check it.

I was taught that a DD3 was a secure brake...in that if you hit the big yellow button only....the coach should never move...even after awhile, even after 2 days.
Keeps them from rolling down hill when an 8 year old pushes them.
The brake pedal application is what IS supposed to release them. FWIW it is supposed to take a 100 psi treadle application.
IF the yellow button alone releases them, they are not working properly.

That's how mine work.

[gus]

My DD3 most times releases when the button is pushed in, sometimes not - I never know for sure since there is no pattern.

It always releases if I push the foot brake pedal.

[uturn]

Well after replacing all the brake valves including the foot treddle valve. The brakes still wont release in cold weather. It was only 55 degrees the other day when I tried. No luck. I'm going to take it to a shop with a pit and hook up some air pressure gauges to see if The parking side is getting too much, Or the releasing side not getting enough. The tr3 valve cluster is supposed to only let 75 psi out. I looked at my records and the first valve I had replaced was a r8 valve because it was leaking. After that once in a while the brakes would not release. I am wondering if the relay might not be working properly all the time. But only when it is cool outside. I am stumped at this point. Any advise or suggestions would be welcome.  Also, I did not replace the lock port T. The mechanic that took it apart said that there was nothing inside the T. On the outside it looks like there might be a check valve inside. The book doesn't describe anything about it. Any comments or suggestions would be welcome. Thank everyone for their replies.   David..

[opus]

Have you tried pushing the button to release the brakes then putting the brake pedal to the floor and releasing it?

[B_K]

What do you get when you drain the tanks?

Just Air, Air/Water, Air/Sludge, Water, Water/Sludge, just Sludge?

If there is any moisture in the system at all it will hamper things when it's REAL cold (freezing or below), but if you have a LOT of moisture (water) in the lines it will make things difficult when cold (even still above freezing).
If there is ANY sludge in the lines it will cause problems in any temperature!

So if you have ever had sludge come out when draining tanks it'd be best to start unhooking air lines and blowing them out to make sure there is none trapped in the lines.

However it still sounds to me like a typical DD3 release problem.
I have chased and chased these problems when it usually turned out to be a "loose nut" between the drivers seat and the steering wheel! (the driver)

But that said when it comes to an owner of a bus. Who has read all the instructions for properly releasing DD3's and is honestly following these procedures. Then ya gotta really start looking deeper.

I am so glad our newer buses got away from the DD3's! Because no matter how many times I would tell/show a driver the proper way to release them older ones w/DD3's it never failed they'd be nodding their heads like one of those little bobbing head dolls, or a Beatles fan singing ya, ya, ya and what I was saying would be going in one ear and out the other. (because face it there was NO WAY it was the drivers fault, that's why they were telling me the mechanic!)
  BK 

[uturn]

I drain the tanks every time I run it. Very little water comes out. No sludge or oil. I have a dryer and I can see it spit a little water, But that's about it. I have tried to release the brakes according to the directions. The full foot application does not matter. When I bought the bus and to this day, When they release, They don't need a foot application at all. When its above 70 degrees, I have no problem, Or when they release, They will release every time for that day. I am going back to the relay r8 and the lock port with gauges and see what the air pressure is releasing and locking the brakes. About the loose nut between the drivers seat and the wheel. That's me for sure. At this point I haven't a clue what to do next. And as a retired truck driver I learned it was always the drivers fault. Please keep the comments and advise coming. Thanks   David..

[uturn]

Well today it got up to 75 degrees. The brakes released right away. I was wondering if there is a way to heat the air coming from the compressor. Like a 12 volt heat probe that I could put in the line safely. Or maybe have the inline heater going into the compressor. I tried a search on the web and couldn't find any heat probes or anything like that. Any comments would help. Thanks  David..

[John316]

.....At this point I haven't a clue what to do next......

How about only driving it when it is warm? Sorry, I couldn't resist.

Seriously though. There is water in the lines somewhere that is probably causing the issues. Where it is? No idea. I really feel your pain. BTDT, with other issues, and it isn't fun.

John

[TomsToy]

When all else fails read the instructions.  From page 4-4 of "da book"
RELEASE OF PARKING BRAKE
A. WHEN EMERGENCY PRESSURE HAS NOT DROPPED MORE THAN 4 psi (27.6 kPa). After applying the parking brake push in handle of push-pull valve.  This applied reservoir pressure to lock port of the actuator thereby releasing the locking mechanism.  At the same time, air pressure is also admitted to the control port of the inversion valve which causes air to be exhausted from the parking diaphragm thereby releasing the brakes.

B. WHEN EMERGENCY PRESSURE HAS DROPPED MORE THAN 4 psi (27.6 kPa).  After applying the parking brake push in handle of push-pull valve.  This applied reservoir pressure to lock port of the actuator thereby releasing the locking mechanism.  At the same time, air pressure is also admitted to the control port of the inversion valve which causes air to be exhausted from the parking diaphragm.  A heavy service brake application will then produce sufficient forward motion of the actuator piston rod to allow the locking mechanism to disengage.  Releasing the service brake application will restore the system to normal running condition.

In plain English, what that says is that if the emergency/parking brake pressure doesn't drop by more than 4 psi that the actuator piston rod is not going to move thus the locking mechanism will not engage and will not require disengagement.  If the pressure does drop, the locking mechanism will engage and will have to be disengaged.  Sounds like some times your emergency/parking brake pressure drops, some times it doesn't.  Also you may not have a regulator if the parking brake is applied with the auxiliary diaphragm because for the same pressure it will only exert 80% of the force that the service diaphragm will.

[TomsToy]

I guess I should clarify, that was in "da book" for an MC-9

[bevans6]

To add to what Tom wrote from the MC-9 manual and to Cliffords comment on the 85 psi regulator...

The parking brake on a DD-3 system is always applied by the parking brake chamber in the DD-3 cannister, never by the service brake chamber.  Two things have to happen - the inversion valve must  be controlled by the push pull valve to remove air pressure from the locking port, thus allowing the push-rod locks to apply, and the inversion valve will also apply 85 psi air to the parking brake port, so that the parking brake diaphragm forces the push rod out.  

If you apply the service brake pedal to "set" the brake, the air pressure in the DD-3 will become the average of what your pedal application sent and the 85 psi the parking brake system sent to the the chamber, based on the relative sizes of the two sections inside the DD-3.  That's because the two sections are only separated by the parking brake diaphragm and otherwise share the same physical space.  That averaged pressure could be higher or lower than 85 psi, no way to tell.

The air that operates the parking brake system comes from the emergency/parking brake tank.  That tank is separated from the rest of the air system by check valves so that the air pressure in that tank goes to maximum pressure (120psi, or whatever your governor high pressure is set to).   It does not rise and fall with the service brake pressure, it just sits at maximum pressure.  That is why there is an 85 psi regulator to limit the pressure to the parking brake system.  Also, the surface area of the parking brake diaphragm is smaller than the service brake diaphragm, so it develops less push from equivalent pressure.  These two things combine to ensure that when the parking brake is released, a full pressure service brake application can overcome the stored pressure locked into the push-rod and release the brake.  The locking mechanism is a sprag clutch with rollers that locks the pushrod, but it requires a tiny movement back into the chamber to actually lock.  That movement comes from air leakage over time, relaxing of the mechanical parts, etc.  If you release the parking brake and that movement hasn't been sufficient to cause the clutch to lock, then the brakes will release without the service brake application.

So the correct way to apply the parking brake is to just push in the valve, and not to also push on the service brake.  If you do happen to raise the pressure on the push-rod over what the parking brake applied all that will happen is it will be harder to release them, you'll need a slightly harder service brake application to get them to release.   If you don't raise the pressure over that point, as soon as you release the pedal application the parking brake pressure will go back to 85 psi and nothing will have happened at all.  If you parking brakes are hard to release I would first find that regulator and check it's pressure set point, check that the inversion valve is working properly and also service the sprag clutch locking mechanism in the DD-3's, but it sounds like yours are now working fine.

Brian

[RJ]

So the correct way to apply the parking brake is to just push in the valve, and not to also push on the service brake.

As a footnote to Brian's excellent explanation, I'd like to address the above statement.

If the system is plumbed correctly, and hasn't been screwed up by somebody else:

Pulling on the parking brake knob sets the brakes.

Pushing in on the parking brake knob releases the brakes, both DD3 & Spring.  DD3s require a full service brake application held for 3-5 seconds AFTER pushing in the parking brake knob in order for them to release properly.

During brake system seminars, Bendix technicians will often answer the question about setting the brake if the vehicle is on an incline.  Over and over I've heard them say that it's OK to set the DD3s if you have to use the service brake to keep the vehicle from rolling before the parking brake is set.  But they emphasize that you should use the absolute minimum service brake application, just barely enough to keep the vehicle stationary.

FWIW & HTH. . .