MC9 Parking Brake Leaking Down While Running

[Cecil The Diesel]

Ok so I just developed an issue a few days ago with my parking brakes leaking down. Its an 82 MC9 8V71N 5 speed manual.

It leaks down while the bus is shut off and while running. It only wants to build to about 60 pounds at idle, and I have to hold the throttle at a high rev to get it to build up enough air to disengage. It was taking a while to build up to pressure before but I thought nothing of it since it stayed aired up fine once I push in the lock to disengage the parking brakes.

I have also noticed a leaking sound coming from around the rear wheels but haven't crawled under it to check it out.

Once the parking brakes are disengaged it airs up fine.

Any ideas?

I'm in Murdo SD for the next few days.

[wg4t50]

Enjoy the car museum at Murdo, if you have not seen it, you will not believe it.  Been there twice.
Air issue, loose fittings, bad pancake on brake chamber, either way, ya gotta get a look see, not a biggie unless you have no tools or parts, why a loose fitting would be a blessing.

[bevans6]

First thought - there is a pressure regulator on the front wall of the passenger side rear wheel well that regulates the pressure of air to the parking brake to 95 psi.  Mine leaked and I had similar symptoms.   Second thought - the parking brake tank is after the pressure protection valve, which explains why the system gets up to 60 psi.  Do not drive the vehicle like this, it is quite unsafe.  The secondary brake system for your bus is the parking brake system.  I would look for the pressure regulator, other things in the rear axle area that could affect this are the inversion valve and of course the parking brake diaghrams in the DD-3 brake chambers and all of the hoses.  On the plus side, if you set the parking brake it will hold regardless of the pressure in the system.

As a side comment, your idea that " I thought nothing of it since it stayed aired up fine once I push in the lock to disengage the parking brakes" indicates that you need to re-think your safety process with regard to air brakes.  A significant change of any sort related to air means you need to find and rectify whatever is wrong.  A lengthy compressor recovery time with the parking brake engages vs not engaged is a huge indicator of a problem that you need to be very sensitive to.

Brian

[Cary and Don]

In finding that leak. A soap bottle is your friend.  Soap every fitting, valve, brake chamber, anything that has air in it.  You will know instantly where the leaks are.  Have somebody apply the brakes and do it all again.  Release the park brake and do it again.  This will give you a good idea as to where the problem is,  and it's amazing how many fittings will be leaking. Hopefully it's not a chamber for valve.

Don and Cary

[wg4t50]

Do not think you will need the soapy to find that leak 

[Oonrahnjay]

     Check valves and relay boxes, too.  I had a quick release valve that was leaking - the diaphragm had gotten old and rotten and there was a hole in it.  I bought a new one rather than taking a chance on trying to rebuild it (<$25). 

[Cary and Don]

Right Dave,  This one is something pretty major.  Betting it's a diaphragm, park brake valve, or relay valve.

Don and Cary

[Gary LaBombard]

Cecil,
I am not sure if this is your problem but this was just what happened recently to a friend of mine.  He also had a very bad leak and asked me to help him find it and without actually getting under the bus it sounded like it was coming from his DD3 drive brakes.  His bus was in dirt, was not at adjusted height to get under and needed to get jacked up to really get under.  So we jacked it up to get to loosen the back wheels which was my plan to take the wheels off to fix without further investigating.  I knew better, I knew better.  Well we could not get the tires off, on either side.  They were just pumped on with a  1" gun to  top stroke so tight that we could not get them off with a 6 ft. pipe on them, not one nut would loosen.  Oh, oh, my plan when to crap.

So we jacked up the bus somewhat more, shored it up and I crawled under it after we got the bus air up.  I heared air leaking a little but a whole lot when my friend applied the brakes, at the passenger side bogie wheel.  Well I was a little more relieved as I did not want to have miy friend spend a lot of money on DD3's and knew the #16 brake chambers on both the front and bogie's were a lot more cheaper.

I removed the one leaking canister, disassembled it and there was a a very rusted canister and a sliced hole in the rubber bladder that let it leak when applied.  This happens very easily on Eagles especially as the Bogie canisters are mounted upside down and allow the dirt, water and debris to get inside very easily around the plunger of the brake chamber, what a design!!!  Well I replaced the bad brake chamber, the one on the driver side as well as if one goes you can bet the other one is on its way or I think that way. 

These buses 30 / 40 or 50 years old wear out.  As cliff said, newer buses are now way, way cheaper, I don't even want to know what I have in mine and it sure won't be a show bus when done.  The part are just out of this world, but if you want safety you got to fix something like a bad air leak or lives may be at stake.  Just posting about it may be a mistake should a accident happen, Please get your leak fixed and drive with confidence you did the right thing.

I am so, so darn tired of finding all the dam things wrong I have on my bus and have no freaking idea when I will ever get to enjoy it, You got no idea how hard it is after all these years to just keep wanting to do the right thing and then see you could of gotten a newer, much newer bus for much much less than what I have in this bus and be able to use it. 
Good Luck
Gary. 

[mcidave]

Gary,
Even though your last statement may be true...We must Drudge Forward and complete our task of completing the conversion-If at all possible.  If not, well we both know we can get a newer bus to absorb all of our free time!

[Gary LaBombard]

Dave,
If at 10 years ago I was at where I am at today I would have been a happy camper as things were still quite good, the value of our old rigs was worth much more than they are now and I was quite younger then too.  But things have happened to ruin my personal dream which I am still trudging to complete.  I still try to influence newbie's to look to newer conversions to buy so they can have more time to enjoy them and quite often use my conversion as an example.  Not to punish my self but offer a tool for an example to consider not to jump into.  Age and health has a lot to do with completing a task as big as these big ole girls which at the time of conception we do not think about, or I didn't.

I certainly don't wish to prevent anyone from having a great nostalgic dream of having one of these great girls to have, I had that dream, still do, but the reality is the cost of parts is so out of this world that if you do not have deep pockets that will not drain into your house funds in time that it becomes a burden then that slows down your success to complete in a timely manner especially when you find everthing needs replacing, Because Of Age.

No one will be the happier to drive his bus for the first time than I after all this time believe me, that tunnel still seems quite long and the light at the end quite small with thoughts like should I pull my engine now or should I??  After all this time, do I go for it like it is or should I put a few more months in rebuilding and a few more thousand in her or more??  When do you draw the line after 11 going on 12 years on things to rebuild / replace?  that is where I am right this moment.  

I just know this Eagle is making me earn all my stripes, hope I make the right decision.

Sorry Cecil, I sort of stole part of your post here, I didn't intend to do that.
Gary

[Oonrahnjay]

I just know this Eagle is making me earn all my stripes, hope I make the right decision.   

    Yeah, I hear you.  I tell people that if I'd known that it would take 1/20 of the work or 1/10 the money, I'da never started.  I guess you should add 1/5th the time.  We'll get there ...

[Cecil The Diesel]

Ok here's where I stand. Replaced the rear air relay valve because at 80 pounds (not sure why I said 60) air was blowing out the bottom. It still won't air up past 82psi even while holding it at a higher rev than high idle. At normal idle its about 72. I called Luke at US Coach and he said its probably a brake can diaphram instead of a relay valve. He told me to pinch off the air lines until it stopped leaking to find the bad can. I did that to no avail so I replaced the valve. What could it be besides the can if its blowing back through the relay?

P.S. when I said I thought nothing of it I meant that since it aired up fine on the non parking brake circut I didn't mean that I thought nothing was wrong. I know something isn't right or I wouldn't be posting trying to find the problem. When I did operate the bus I kept constant check on the air pressure. It's parked now until I fix this issue.

[bevans6]

Do you have air pressure in the parking brake tank?  It's the one closest to the passenger side on the rear wall of the front axle bay.  It should have air pressure even if other tanks have dropped to zero.  I think you will find that it doesn't have pressure.  I think what is happening is that when you park and put the parking brake on, the air for the parking brake comes from the parking brake tank.  It goes to the pressure regulator, then to the inversion valve and then to the brake cannister.  You probably have a leak in one of the parking brake diaphragms, in the pressure regulator, or in the inversion valve.  The parking brake diaphragms are the back chamber of the DD-3 brake cannister, the pressure regulator in on the front wall of the rear axle bay, and the inversion valve is on the differential (it's a six sided valve with several hoses going to it.   The hoses involved could equally be bad, of course.  When this same symptom happened on my bus it was the 85 psi pressure regulator that had failed.  The reason it happens around 80 psi could be related to the actual pressure regulator set point, or it could be related to the pressure protection valve set point.  If the parking brake system is leaking down, it won't start to get air again until the system pressure gets past the PPV setpoint, which is normally around 65 psi but could easily be closer to 80 psi.  When the pressure protection valve opens at 60 - 80 psi, air is released into the parking brake system and is then leaking out.

It is probably a bad can, to be honest.  If you want you could take the hose off the middle connection on both cans, that would let you know which can is releasing air out as the pressure rises.  When the parking brake is on air should be present at the rear connection, and no air pressure at the middle or the front connection.  New DD-3 cans are available at Rebuilder Enterprises Inc in Chicago.  They are pricy but they are also new or freshly rebuilt, depending on which you choose.

Brian

[Oonrahnjay]

... It is probably a bad can, to be honest.  ...

    No experience with DD3's but there seem to be a couple of kinds of leaks.  One is where there's a place where air is escaping from the system from a loose connection or bad part (diaphragm, seal, etc) inside a component.  That one is usually easy to find with soapy water.  The other kind is where the leak is internal and the air doesn't come out where the actual bad or leaking part is.  Parking brakes are one of these - if an internal diaphragm on a (normal) dual can is damaged, often the air which leaks doesn't come out of the can, it comes out of a relief port on a valve in the system somewhere else.  Brake circuits are also problematic.  Since they work "backwards", (i.e. pressure for no brakes and relief of pressure for brakes on), if there's a leak that's causing an internal reverse flow, you'll often find that the leak causes the parking brake to be applied if it's on that side of the hand valve on the dashboard but if there's a leak in the hand valve itself, sometimes it will hold the parking side off but leak air from the "feed" out the valve itself.

    Having the engine running, the parking brake released, and getting under to find a leak is often a dangerous combination, but sometimes you have to check for leaks in lots of different configurations -- parking brake set, parking brake released, someone else stepping on the brake pedal etc. to see if you have a leak in only one condition.  Mostly air that comes out a relief or exhaust port on a valve should be momentary - it should only have air coming out of it when something is changing (the person in the driver's seat takes his/her foot off the brake, the parking brake is set, etc.) so if you have a steady stream of air out of a relief port -- a place where air should escape sometimes -- that may point you to the problem.  But don't forget that the actual problem may not be right there, it may be air that's "backfeeding" or leaking from somewhere else and going into the place where you can detect air coming out.

    Sometimes it's not easy but a good logical inspection and diagnosis usually points you towards the bad part.

[Cecil The Diesel]

I have two reputable diesel mechanics working on this with me and can't wrap their heads around why a bad diaphram would blow back through the relief. We've spent the last three days under the bus. The only issue is that these guys are not familiar with how the dd3's work. I'm not either but about to get real up close and personal. It's a spring actuated park brake can right? How do you capture the spring and what is the disassembly procedure?