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Author Topic: Parking brake does not release all the time  (Read 3133 times)
uturn
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« on: September 29, 2013, 04:09:40 PM »

I have a 1977 p8m4905a. The parking brakes wont release every time. When its cold or after all air is leaked out, About a day. The brakes will blow off the relay. The brakes will release after some time. I turned up the compressor governor. The cut out was 120 psi. I turned it to 135 cut out. The dash gauge is accurate. I replaced all the rear brake valves. The LQ2,R8 double checks and chambers and hoses. The book claims there is a standby valve that is a tr3. I have 3 tr3's on the back. Any comments would be welcome. Thanks, David..
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opus
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« Reply #1 on: September 29, 2013, 04:47:15 PM »

Sounds like a brake stuck to the drums?
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« Reply #2 on: September 29, 2013, 05:08:50 PM »

Check the regulator they require 85 lbs max to set anything over 85 lbs the DD3 can be a bear to release
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Nick Badame Refrig/ACC
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« Reply #3 on: September 29, 2013, 05:26:20 PM »

Hi David,

That happens to me once in a while. After releasing the parking brake, I just push my service

brakes all the way to the floor then release and the parking brake will also release.

Must be something hanging up be this helps in a pinch.

Nick-
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uturn
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« Reply #4 on: September 30, 2013, 04:21:14 PM »

Thanks for all the replies. Where is the regulator at? I have bled the air off to 60 psi before and set the brake with the same results. I have tried a full foot application to release the brakes and that doesn't work. The brakes release on their own with out it. But when they don't, I try that also. Once they release the first time they will release every time until it sits a day or so. I'm taking it to a shop this week that has a pit to look for the standby valve that is shown in the manual. It states that the valve is related to the foot application valve and sends air to the parking relay valve. I haven't tried it since I turned up the governor on the compressor. That will be tomorrow. Thanks again. David..
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RJ
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« Reply #5 on: September 30, 2013, 05:28:21 PM »

David -

First, you don't need the system pumped up to 135 psi.  It's designed to work at 120 - 125.  So back that off.

Second, what type of rear brake cans do you have on your Buffalo?  Are there two or three air lines going to them?  Some coaches came with DD3s, others of that vintage came with spring brakes.  Makes a difference in the release procedure.

Let us know what you've got so we can better help you.

FWIW & HTH. . .

 Wink
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RJ Long
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« Reply #6 on: September 30, 2013, 06:43:05 PM »

I try to set my DD3 no more than 1/3-1/2 foot pedal so they are easy to release, never push all the way.

Mine usually release on their own when I push in the knob if I wait a few seconds.
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« Reply #7 on: September 30, 2013, 10:42:12 PM »

David & Gus -

As a former coach operator instructor, here's the proper procedure for setting and releasing the parking brake:

http://www.busconversions.com/bbs/index.php?topic=26449.msg290100#msg290100

FWIW & HTH. . .

 Wink
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RJ Long
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uturn
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« Reply #8 on: October 01, 2013, 01:26:28 PM »

RJ, Thanks for the input. I have 3 lines going to my brake chambers. I looked in the manual about the air pressure and it reads that the cut out pressure should read on the dash gauge 123 to 131 psi. It was cutting out right at 120. And the cut in was about 90 psi. The book reads it should be between 102 and 108. Now it is cutting in at about 103. I released them today after building up air from zero to 135 and they released right away. I will turn it back down a little after I return from the shop tomorrow. I am going to replace the front service valve although it is still working it looks like it has never been replaced. And I'm going to look for the standby valve that is supposed to be in the front area somewhere. The manual reads that the standby valve relays pressure to the parking valve after you have applied foot valve.  I am by no means an expert on this bus. I drove trucks for 27 years and have had to rig blown chambers and air leaks to get me home before. A dime in the air line and backing off the brake and so on. This is a totally new thing for me. Thanks for all the replies and keep them coming. David..
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« Reply #9 on: October 01, 2013, 02:29:25 PM »

RJ,

I don't really understand pushing the service brake all the way and holding it when mine release 99% of the time with no SB use at all?

What am I missing here?
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PD4107-152
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« Reply #10 on: October 03, 2013, 02:05:00 PM »

Gus -  when RJ came out to teach us the ins and outs of buses we found that when we pushed the brake pedal all the way down and held it - we had a leaky treadle valve that needs repair.  I believe the answer to your question is to check and make sure that all air lines will hold at maximum pressure for 3-5 seconds.

(What say you RJ?  Did I actually learn something from your 3 day course Wink )

-Sean


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« Reply #11 on: October 03, 2013, 06:24:29 PM »

Makes sense!
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uturn
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« Reply #12 on: October 06, 2013, 04:13:53 PM »

Well I took it to the shop and looked for the standby valve. No where to be found underneath the front end. The brakes have been releasing and I turned down the governor to about 125 psi. I checked the foot valve and did hear a leak at a full application. Could this affect the parking release? It always has released with out the foot pedal before. I will get the foot valve replaced soon though. Not much of a leak and only when the pedal hits the floor. Thanks for all the help and replies.  David..
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uturn
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« Reply #13 on: November 07, 2013, 04:16:16 PM »

Well I had the foot valve replaced today and the parking brake still wants to stick. Not all the time though. The mechanic at the bus shop told me that he has seen a lot of buses, Most of them MCI.s that had the governor set at 140 psi because the brakes wouldn't release below that. It is hit and miss with this bus. I was wondering if a heated air dryer would help. I have a dryer on the bus and it works fine. But there isn't a wire going to it, So I would guess it isn't heated. In cold weather is when it is the hardest to release. Not real cold weather either. I'm in Florida and below 60 degrees it sticks. Any thoughts would be welcome. Thanks, David.
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Dave5Cs
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« Reply #14 on: November 07, 2013, 06:55:36 PM »

I have mine set at 118, that what both my books say and it releases  every time.
Maybe pull the push-pull valve apart and either replace o rings or at least lube them up with non rubber type grease.
Are the rollers in the cans worn or the "s" Cams.
Dave5Cs
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