MCI Parking Brake Question - Update

paul102a3 · August 22, 2009, 04:49:21 AM

The other day I was backing into a parking space, set the parking brakes, removed my foot from the service brakes and the bus started to roll backwards.

I was in reverse gear at the time so I set the trans in neutral, reset the parking brake, shifted into reverse to double check and the bus moved a few inches and then stopped.

Since the episode, I have noticed the parking brakes do not hold well in reverse. I can overpower them with a little throttle while in reverse gear. The same is not true in forward gear. I have given a fair amount of throttle in forward and the parking brakes seem to hold fine.

Where do I start to diagnose the problem?

The service brakes seem fine and I have not noticed any change in braking distance.

Paul

JackConrad · August 22, 2009, 05:05:08 AM

Sounds like you need to adjust your brakes. Brakes do not have as much holding power in reverse and will fail to hold in reverse before they fail to hold going forward. Jack

Dreamscape · August 22, 2009, 05:06:49 AM

Do you have automatic slack adjusters or manual?

I agree with Jack, maybe the slacks need adjusting.

Paul

John316 · August 22, 2009, 05:08:20 AM

Interesting. I don't have much to say about your problem.

FWIW we can overpower our parking brakes. We hate to do it, but after we first got our bus, we got back, parked the bus, and the next day found that there was water in the lines, and the lines to the parking brakes had frozen. We, of course, tried everything (remember we didn't know ANYTHING at all about the bus). We were on the phone with Junior, and everything. I won't go into the rest of the story, but we also had the gasket that is between the oil cooler, and the engine fail. We poured a gallon of oil in every ten minutes, while we were driving to take the bus to the shop (it was pouring out!). We had a forty five minute drive...

So anyways, we used to be able to over power them, but that might have changed after we rebuilt the undercarriage.

God bless,

John

luvrbus · August 22, 2009, 05:29:51 AM

Paul, the dd-3 parking brakes sets at the air pressure you have built up in the system try 110 0r 115 lbs before setting the parking brake.
Ah thank heaven for spring brakes.

good luck

paul102a3 · August 22, 2009, 05:33:19 AM

I have no idea if the slack adjusters are manual or automatic. How can I tell?

I had ABC bus do a pre-purchase inspection of the brakes and suspension and they did adjust the brakes at that time. This was in April of this year and a little over 2,000 miles ago. Since they had to adjust the brakes, would that mean the slack adjusters are manual?

Paul

paul102a3 · August 22, 2009, 05:38:23 AM

Luvrbus, Would it be a good practice to wait until the system purges before setting the parking brakes?

Paul

Dreamscape · August 22, 2009, 05:44:24 AM

Automatic slack adjusters look like this.

Check your manual, also it might be stated what type on your workorder.

Paul

Busted Knuckle · August 22, 2009, 05:46:33 AM

Paul,
Yes it would be a good practice, but should not really be necessary!

And FWIW I would bet that you have manual slack adjusters or froze up and worn out auto-slack adjusters!

What yr is yer bus? (not that that really means anything I have seen OLD buses with autos and new buses with manuals)

I agree it sounds like the brakes definitely need adjusted, and/or the brakes may need replaced!

HTH

BK

luvrbus · August 22, 2009, 05:47:18 AM

Paul, auto slack adjusters can be adjusted manually it is a good idea to do it every so often anyway to check for the ones not working.
Your model bus came with auto slack adjusters.
Slack adjusters mean just that they take up the slack to adjust the brakes properly do it the old fashion way like ABC did for you

good luck

Dreamscape · August 22, 2009, 05:52:40 AM

This video will give you a pretty good understanding about adjusting slack adjusters.

Just be safe when underneath, block, chock and lock, put the key in your pocket, turn off batts.

Adjusting air brake system slack adjusters

luvrbus · August 22, 2009, 05:58:22 AM

Paul, our instructor at the Bendix school recommend setting the DD-3 at 110 to 115 lbs because it takes 1 lb more of air pressure to release brakes from the set postion and as they wear a little more.

Ps he was not a big fan of the DD-3 parking brake and I am sure he is jumping with joy since Bendix stop make the system

good luck

John316 · August 22, 2009, 06:30:48 AM

Clifford,

So does that mean that they don't have the "spring applied air released," system anymore, or does that just mean that they stopped making the DD3, but still have that system.

Thanks.

God bless,

John

paul102a3 · August 22, 2009, 06:38:05 AM

Thanks everyone for the info. The YouTube video was real helpful. I looked through the book last night and thought I understood the procedure but to see the process in action really puts things in perspective.

I think I will take the bus back to the shop and have them look things over. We are leaving in two weeks for a rather long trip up to Canada and then back down the coastal towns of New England and the mid Atlantic states. The last thing I need or want is a brake problem on the road. Being a rather safety conscious person, I would rather leave with the knowledge that the brakes are working as close to 100% as they can be.

Paul

luvrbus · August 22, 2009, 06:50:05 AM

John, the spring brakes are what is used today the DD-3 are gone.
I just looked at my notes taken during class and can see why he didn't like auto adjusters or the DD-3 here is the info he gave us fact or fiction I don't know because I have spring brakes

Rebuild kit for a DD-3 brake has 70 parts
rebuild kit for a spring brake has 9 parts
auto adjusters have 25 parts
manual adjusters have 8 parts
serviceable parts

good luck