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Author Topic: Parking brake release  (Read 1469 times)
Emcemv
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« on: September 17, 2013, 07:16:47 PM »

So, I adjusted the brakes this weekend, all now have a 1" stroke. I moved the bus today and had trouble getting the brakes to release when I push in the parking brake knob. I have rad the manual and am aware that I need to do a heavy brake pedal application after releasing the parking brake to get them to release. After several attempts, they finally released and I was able to move ok and now everything seems to work fine. In reading the manual, it seems that maybe the inversion valve could cause this problem.  I have a spare one of those, maybe I should give that a try.  It looks a bit tough to change, lots of hoses to keep track of.

Any thoughts or comments?

Bruce
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Bruce & Nancy Fagley
1973 MCI MC-7 Combo Freighter
450HP DD 8V-92T 2000 Reman
HT 740 Allison
Woodbury CT.
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« Reply #1 on: September 17, 2013, 07:59:08 PM »

Bruce -

Sometimes with DD3 brakes they'll hang a little after adjustment or if there's too much service brake applied when the parking brake is activated.  They'll also hang if it's been awhile since the rollers inside the cans have been serviced.

I've written about this before, but it's good to repeat it now and then as a refresher:

Setting DD3 & Spring Parking Brakes:

1.  When parking and setting the parking brake, once stopped, release service brakes to the point they are just barely preventing the bus from rolling.

2.  Pull parking brake knob to set brakes.

3.  Release service brakes, but cover brake pedal in case parking brake fails to hold.

4.  Once secure, continue normal shut down procedures.

Releasing DD3 Parking Brakes:

1.  Start engine.

2.  Build air pressure to governor cut out point (about 120 psi +/-)

3.  With foot lightly on service brake pedal, push in parking brake knob to release brakes.

4.  Once knob is pushed all the way in (NOT BEFORE!!), make a full, to-the-floor service brake application, and HOLD it for 3 - 5 seconds.

5.  Release service brakes, coach should roll easily.  Be sure to cover brake pedal "just in case!"

6.  If brakes do not release,  repeat steps 2 > 5.

Releasing Spring Brakes:

1.  Start engine.

2.  Build air pressure to governor cut-out (about 120 psi).

3.  Cover service brake.

4.  Release parking brake by pushing "in" on knob.

5.  Coach should roll, but continue to cover service brake for safety.


Doing a daily pre-trip inspection is important for the safety of you, your passengers and other motorists.  Learn how, especially the air brake steps - they're the biggest piece of SAFETY equipment on your coach!  If you need a copy of a good generic pre-trip, you can find one here:

http://busnut.com/forum/index.php?action=articles;sa=view;article=40

FWIW & HTH. . .

 Wink

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RJ Long
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bevans6
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« Reply #2 on: September 18, 2013, 03:30:37 AM »

I would make sure I had greased the DD3 cannister rollers, there is a grease nipple and only inject a small amount of grease.  The rod is hanging because the rollers aren't letting go of the rod.  Could be insufficient stroke to get them to disengage, could be the rod is worn differently at the new place they are gripping it.  If it persists I would try adjusting the stroke to 1.5", 1" is pretty tight.  On my MC-5 there is a pressure regulator for the parking brake pressure that is on the front wall of the rear axle bay.  The regulator should be set to 85 psi, if it is set too high or not working the parking brake could be getting set on too hard so the service brakes can't release it properly.

Brian
« Last Edit: September 18, 2013, 03:32:22 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Emcemv
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« Reply #3 on: September 18, 2013, 04:15:51 AM »

Thanks Brian and RJ......

I am thinking that the stroke is a little too short, will open them one click. I did grease the canisters and everything else on the brakes.

RJ, thanks for the procedure on setting the brake, I will print that out!!
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Bruce & Nancy Fagley
1973 MCI MC-7 Combo Freighter
450HP DD 8V-92T 2000 Reman
HT 740 Allison
Woodbury CT.
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« Reply #4 on: September 22, 2013, 07:33:27 PM »

Brian & RJ

I backed off the adjustment to 1.5" stroke and followed RJ's procedure and the brakes worked perfect on our trip to Newport RI this weekend. Thanks again to both of you for the help.

Bruce
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Bruce & Nancy Fagley
1973 MCI MC-7 Combo Freighter
450HP DD 8V-92T 2000 Reman
HT 740 Allison
Woodbury CT.
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« Reply #5 on: September 22, 2013, 09:29:06 PM »

RJ,

I had the experience twice where I set the DD3 brakes, and the bus began to roll.  Since that happened, I give the service brake a good application after the DD3 is set, and have not had it roll again.  Is this procedure good, bad or totally irrelevant.
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« Reply #6 on: September 22, 2013, 09:31:58 PM »

I had a freaky thing happen to me, maybe not your problem but I will throw this out there for you anyhow.  My parking brake push / pull out valve PP1 on the dash was brand new, only the actual valve handle that you push in would not push in as it was bigger than the orifice it slides in, maybe about .010 in diameter bigger.  I went nuts, almost started taking the system apart etc.  Anyhow, fooled around and looked closer at the dam valve and took the yellow handle off and pushed in the center the yellow handle attaches to, and it pushed right in when the air pressure was up over 60 psi!!.  this may not be your problem, just wanted to let you know of mine, the system was new and what in hell do I know but freaky stuff happens and it sure does to me.  Hope it is this simple for you.
It sounds like you did not replace this so I am way off base here, good luck anyhow. These guys will help you.
Gary
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« Reply #7 on: September 22, 2013, 10:34:02 PM »

I had the experience twice where I set the DD3 brakes, and the bus began to roll.  Since that happened, I give the service brake a good application after the DD3 is set, and have not had it roll again.  Is this procedure good, bad or totally irrelevant.

Lin -

Sounds to me like the rear brakes are out of adjustment.

You also run the risk of setting the locking rollers so tight you won't be able to release the brakes if you continue to set the parking brake them that way.

FWIW & HTH. . .

 Wink
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RJ Long
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Emcemv
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« Reply #8 on: September 23, 2013, 04:20:59 AM »

RJ

Am I understanding this correctly that one should not make a hard service brake application while the parking brake is set?


I'm going to print all this out and paste it on the dash!

Bruce
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Bruce & Nancy Fagley
1973 MCI MC-7 Combo Freighter
450HP DD 8V-92T 2000 Reman
HT 740 Allison
Woodbury CT.
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« Reply #9 on: September 23, 2013, 04:44:44 AM »

On an MCI bus like mine, there is an 85 psi pressure regulator that limits the air pressure that goes to the inversion valve to set the parking brake.  Also, the parking brake diaphragm is a little smaller than the service brake diaphragm so it develops less "push".  This is to ensure that the service brake can always overcome the pressure of the parking brake and effect the release.  If your parking brake doesn't reliably set, you could look into that pressure regulator and the inversion valve to make sure they are switching things correctly.

As for the big service brake application after the parking brake is set, that was a research project for me a little while ago.  I wanted to figure out exactly what would happen.  This is what I came up with - unlike spring brake chambers, which are two separate systems operating on the push-rod independently and need an anti-compounding valve to prevent simultaneous application of both spring brake and service brake, DD-3 systems do allow simultaneous application of parking brake and service brake.  What happens in that case is this:  The parking brake and service brake share the same physical space inside the chamber and push on the push-rod in exactly the same way.  The area's are separated only by the rubber diaphragms.  If air is in only the service section the parking brake diaphragm lays along the back wall of the chamber and is not in effect.  If air is in the parking brake section the service brake diaphragm lays forward and is not in effect.  If air is in both chambers, the working pressure will be an average of the pressures in the two sections based on the volumes that they have.  In any case, the pressure will be lower than a full service brake application with the parking brake off, so the service brakes should always be able to release the parking brake.  And that is what actually happens, because I've tried it...   Grin

Just the pressure in the parking brake system should always set the brake.  If you also do a medium or light service brake application where the pressure low, nothing happens because what you've actually done is momentarily lower the average pressure and that does nothing with the brake set.  If you do a 100 psi service brake application, then you've momentarily increased the average pressure in the can from 85 psi to maybe 93 psi, and so your next 100 psi application when you want to release the brakes should move the pushrod and release the sprague clutch on the pushrod.  But no, it's not a great idea to set the brake harder with a full service brake application.  The system wasn't designed to work optimally that way.

Edit:  If there is anything wrong with the parking brake system such that you got the lock release set to lock via the inversion valve, and managed to get a full service brake application when the parking brake diaphragm was empty (leak, bad seal, whatever) that brake won't ever release until you take it apart. 

Brian
« Last Edit: September 23, 2013, 04:51:35 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
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« Reply #10 on: September 23, 2013, 08:35:06 AM »

RJ- Just to clarify, this rolling when I set the parking brake happened twice in a 5 year period.  It is possible that something was stuck or out of adjustment.  It has been a couple of years now since I started this service brake application upon setting the parking brake, and the roll has not happened again.  I will now experiment without doing it and see if anything changes.  Thanks

Brian-Thanks for the detailed explanation.

Bruce- Sorry about this hijack, but it seemed close enough to not need a new thread.  Thanks
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Emcemv
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« Reply #11 on: September 23, 2013, 09:22:59 AM »

No hijack at all Lin......I need to know everything I can about the brakes....this is pretty important stuff!!  Thanks for all the replies!

Bruce
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Bruce & Nancy Fagley
1973 MCI MC-7 Combo Freighter
450HP DD 8V-92T 2000 Reman
HT 740 Allison
Woodbury CT.
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« Reply #12 on: September 23, 2013, 06:25:24 PM »

Hmmm. . .

Maybe it's time to resurrect this old thread:

http://www.busconversions.com/bbs/index.php?topic=1489.0

It's pretty long, keep reading. . .

 Grin

 
« Last Edit: September 23, 2013, 06:32:41 PM by RJ » Logged

RJ Long
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« Reply #13 on: September 24, 2013, 11:22:27 PM »

Bump
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RJ Long
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