Parking brake doesn't always release

[Fredward]

On our MC-5 with DD3 the parking brake doesn't always release.
Push in the button, press the brake pedal to the floor, hold it, count to TWO and release. This sequence usually works. Sometimes you push in the button and they release without doing anything else. But sometimes they won't release without me going through the release sequence several times. I think one or the other of the cans is sticking, but can't be sure because I can't really see.

I figure it could be one of three things: the parking brake button (valve) itself, the inversion valve above the rear axle or the little rollers in the DD3 sticking. Two years ago I took both DD3s apart and put new rollers in with a little bit of lube. Also replaced the parking diaphragm in them. But that didn't clear up the problem. They still stick occasionally. But when they release they release fully and everything's good.

Where should I start?

[Frank @ TX]

Hi Fredward,
I think everyone's brakes will do that sometimes.
All I do is wait until the air pressure is at least as high as it was when I applied the brakes in the first place.  Hope this helps.
Frank

[eagle19952]

Hi Fredward,
I think everyone's brakes will do that sometimes.
All I do is wait until the air pressure is at least as high as it was when I applied the brakes in the first place.  Hope this helps.
Frank

Which in my case means if I am on flat ground...I apply my Park brake "sparingly yet adequately"...

[Brian Diehl]

Fred, have you checked the pressure regulator for the parking brake circuit?  Also, the parking brake is supposed to be lubricated regularly and J.D. recommends using a lithium grease mixed with a little bit of ATF to make sure it flows nicely even in the real cold weather.

[eb99603]

Hey Fred, I'm in the early stages of a similar issue. I just noticed a 6" tire mark on the pavement from where I pulled away from the curb earlier today.  When mine have stuck, a quick trip in and out of reverse usually frees them.

[bevans6]

When you apply the parking brake, just use the push pull valve, don't step on the brake pedal.  They are supplied with 80 psi air from the parking brake regulator via the inversion valve.  The pushrod needs to move out slightly to release the sprag clutch (the rollers).  Since the pressure from the brake application to release the parking brake is pushing on the brake shoes through the pushrod and the brake mechanism, make sure that the slack adjuster is adjusted correctly and measure the pushrod movement.  If you have over 2" of pushrod movement you need to adjust the brakes.

The reason the parking brake can sometimes release just using the push pull valve and not doing a service brake application has to do with how the sprag clutch works.  When you apply the parking brake 80 psi of regulated air is supplied to the parking brake section of the DD3.  This pushes out the pushrod and applies the brake.  As long as there is over 80 psi of pressure in the emergency/parking brake air tank, the pushrod will stay out under pressure.  The sprag clutch that locks the pushrod in place will not engage until the pushrod moves back into the cannister a small amount as air pressure leaks down and the air is not pushing quite as hard on the parking brake diaphragm.  Until that happens the rollers are touching the push-rod, ready to lock, but they have not actually locked.

Brian

[Fredward]

Brian,
I did take both DD3s apart and installed new rollers along with the grease/ATF mix prescribed by JD two years ago.

Bevans,
I know what you're saying and usually don 't have any pressure on the foot brake when using the push pull valve. And thanks for explaining why they sometimes release without a service brake application.

It's just makes me nervous to be 2,000 miles from home and having difficulty getting the brakes to release. If I start swapping parts, I'm not even sure where to start!  And if I do nothing, what are the chances of a DD3 staying permanently applied requiring wheel removal and service work?

[bevans6]

What are the chances?  Only time can possibly tell.  It's a pretty foolproof system if it's all new, but the most probable thing that could cause it to stick, if the rollers are new and free in their little house, might be a worn pushrod.  Can't inspect without disassembly.  If they do stick on, you can still take the cannister off by just undoing the two nuts that hold it on.  It gets pushed back by the slack adjuster, but there is room to get it loose and get the clevis pin out.  If it was mine, I would jack it and block it, take the rear wheels off so I could get up close and personal with the cannisters on both sides, I'd measure the air pressure after the 80 psi regulator, then I would measure the pushrod extension at 100 psi application pressure of the service brakes to make sure it was in spec, if out of spec I would adjust if they are manual slacks or get a mechanic certified on air brakes if they are auto-adjusting slacks to find out why they aren't in spec.  With all that sorted, I would get air pressure up to 120 psi, apply the parking brake, observe how far out the pushrod goes, drain the emergency brake tank (probably have to drain all the other tanks too) to force the pushrod lock to apply fully, then get air pressure back and try releasing it.  It should not release with just the push/pull valve operated, it should release with a single full service brake application (just on/off, not held for three seconds or anything else, just on/off as fast as you please).  Get someone else to do that while you watch one of the cannisters to see what happens - should be a tiny flex towards the slack adjuster as the pressure of the service brake application comes on, then a big relax as the springs pull the pushrod back inside the cannister.  Then repeat the whole process for the other side, so you can observe it applying, locking and releasing.  If you do that process in that order, you'll verify proper operation of everything.  If the lock releases with a prolonged application of three to five seconds, but doesn't with a real quick on/off, then it's sticking more than it should be and probably needs some love.  

Brian

[eagle19952]

 ..good answer

[bevans6]

It occurred to me to say why you should measure the pressure after the 80 psi pressure regulator.  The reason is that the air to that regulator comes from the emergency/parking brake tank.  That tank gets it's air from the main air system through a check valve.  That means that it fills up to the maximum air pressure in the system, usually 120 psi, and stays there all the time even while the main air system pressure goes up and down while you drive the bus.  The emergency tank is always full at maximum pressure.  If the pressure regulator fails open (pretty rare, they usually just leak and make a loud noise) or if some handy person screwed the adjusting screw in all the way, you would have a situation where the parking brake gets applied with full air pressure and it might be very hard to get it to release.  The nice folks at Bendix thought of this, and made the parking brake diaphragm size roughly 20% smaller than the service brake diaphragm size, so it puts out around 80% of the push than the service brake does for the same amount of air pressure.  So a full service brake application at 120 psi should always trump a parking brake application.

Brian

[Fredward]

Brian thanks for all the detail. I am planning to check the parking system pressure to be sure its at 80 psi and then go from there. Interesting thing is I think only one is hanging up and it happens so seldom that I can't reliably reproduce the symptom with the wheels off. But measuring the stroke of the push rod makes a lot of sense. I understand your trouble shooting sequence and that makes sense. Thanks.
Fred

[Fredward]

So the rods travel about  1.5" from release to full application. I think that's ok. And the angle between the slacks and the rod is 90 degrees or more. 

But the parking circuit is at 100 psi which is a problem. From looking at the schematic, I think the regulator  is up in the front axle area with the two tanks. The MC5 is different from the 7 8 and 9 I'm guessing. Does anybody know if that is the case? I know its not in the rear axle area.

I'll locate the regulator and turn it down to 80 psi I guess?

[bevans6]

On my 5C it's in the rear bay near the passenger side height control valve.  One hose leading to the inversion valve on the differential, one input hose.

[Brian Diehl]

Fred - the regulator should be in the back axle area.  On my bus it is supposed to be set to 85 psi according to the manual.

[luvrbus]

My 5 would do that Fred and I changed about every valve and regulator,I finally just started pumping the brake till the pressure would be around 85 lbs then set the parking brake was a real pita but worked for me