MC9 Parking Brake Leaking Down While Running

[Rick 74 MC-8]

Cecil
I believe where you are mistaken There is not a caged spring on a DD 3. Look at it instead of a caged spring as another air diaphragm With a separate source of air. I hope that helps

Rick

[Cecil The Diesel]

So what's the disassembly procedure to install new diaphragms?

[Rick 74 MC-8]

I have no clue but I'd sure it involves a hammer and a lot of cuss words.

Rick

[bevans6]

The DD-3 chamber has three air ports and two diaphragms.  You can get the DD-3 manual from Bendix by googling DD-3 brake chamber.  Basically the port closest to the push rod controls the push rod lock, the center port is for the service (regular) brake section and the rear most port is for the parking brake section.  In normal operation there is pressure at the front port to hold the push rod lock open, and service brake pressure in the center section to operate the brakes.  There is no pressure in the rear section so it's diaphragm just lays flat around the inner surface of that part of the chamber - in effect it is the back half of the service brake section.  When the parking brakes are on, air is removed from the front port so the push rod port locks and holds the push rod out, there is no pressure in the center service section, and there is 85 psi pressure in the parking brake section.  Since the only thing between the parking brake section and the service brake section is the rubber parking brake diaphragm if it has a leak, it will leak out the service port.  The service port is connected to the R-8 valve and the bottom of the r-8 valve is the quick release valve for the rear brakes.  It is designed to seal up and pass air to the service brakes on a normal brake application but open and allow any air in the service brake chamber to exhaust quickly as soon as the application is released.  That is why you hear the leak at the R-8 valve - it's job is to exhaust any air trapped in the service brake chamber, and in this case you are getting air leaked in there from the tiny hole in the parking brake diaphragm.

I strongly recommend buying a new or rebuilt chamber - when I took mine apart (and mine were old but still operating perfectly) there were no rebuildable parts at all.  Even the castings were junk.  Still working, but not good enough to rebuild.  In any case, all you do is take the chamber out of the bus, undo the clamps and it should come apart.  There are some springs inside it but they are not heavy springs so there is little danger of it springing apart and hurting anything.  Download the manual - it has the instructions in it.

http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&ved=0CEAQFjAD&url=http%3A%2F%2Fwww.bendixvrc.com%2FitemDisplay.asp%3FdocumentID%3D2393&ei=AtUUUou1IfT_4AOr5YGQCA&usg=AFQjCNE4SDqaaZ1QkKIpClTgKDoobp6I8Q&bvm=bv.50952593,d.dmg

Brian

[Rick 74 MC-8]

Excellent explanation Brian I kind of knew the theory behind them But not enough to explain it

Rick

[bevans6]

Thanks for the nice comment, Rick, I appreciate it.  I am quite obsessive and probably over the top about air brakes.  As soon as I took my air brake course to get my driver's license I figured out that unlike hydraulic brakes, which basically work by muscle power and passive mechanical leverage, air brakes are an active system that requires the creation and harnessing of air pressure to work at all, and the miracle is that they work as well and as reliably as they usually do.  So I had to quite obsessively figure out and learn exactly how all of the systems in my air brakes work, and how to troubleshoot and test every single component.  It took a long time and with my memory the way it is, it's an ongoing process - I forget things and have to figure them out again...

Brian

[Cary and Don]

We have changed the diaphragms in our DD3 chambers.  They were not the original chambers and were in good condition other than the rubber.  We did them without removing from the bus.  You don't even need to remove the air lines. It was really easy.  Just make sure all the surfaces are smooth and clean when putting back together so there won't be any leaks.  We also went by the theory that if you change one,  you change all of them.  When you price those chambers you will see why you will want to check them before replacing them.

Don and Cary

[bevans6]

MCI's have them installed in a stupidly tight area, you barely have enough room to put them in and out.  If you don't remove them you can't service the locking section, which if your unit is old you might want to do.  The rear brakes of an MCI is not the triumph of design and function that the rest of the bus is...   

Brian

[Cary and Don]

Brian,

Maybe they used the same engineer that put the slack adjuster on our Eagle one inch from the rear end.  LOL

Don and Cary

[Cecil The Diesel]

Thanks Brian for the excellent explanation. I understand whats going on now. I guess I'll see if I can get some cans expedited to me in Cripple Creek CO.

Thanks everyone for the help. This is an amazing community to be part of.

[Cecil The Diesel]

Has anyone tried to swap to a 30/30 spring brake can on an MC9?

[bevans6]

Luvrbus claims to have done it, or to want to do it, or to have a plan to do it...     It would involve basically replacing the entire rear brake spider assembly to rotate the cannister mounting to a place where it didn't interfere with the rear spring bags.  In short, basically no one has done it that I know of.   It would involve far more heavy fabrication that I would ever contemplate.  I guess you found out how much DD-3's cost, eh?  It's just one of the costs of running a bus that no one tells you about before you buy a bus...  I took a while to decide, but in the end I bought the new cans.

Brian

[Cecil The Diesel]

I ordered new diaphragms from Luke earlier today for the DD3's. hopefully my cans are decent enough to pop them in and go. Luke suggested going this route so I'm crossing my fingers, and hoping for the best. I thought if it was an easy setup to go with springbreaks I would go ahead  and modify but being that I'm pretty much sitting stranded in a parking lot in Cripple Creek CO that I'll go with the easy solution first. Wish me luck.

[bevans6]

Well, I support outlaw country, and I do wish you luck!  I pull the rear wheels to work on the brakes, and if you can't get enough air pressure to release the parking brakes you can undo the cannister mounting bolts and relieve the tension on the pushrods that way.  Check that your hoses are OK.  If the lock mechanism is working don't take it apart, just grease it lightly.  If you need to release the lock on the pushrod, just apply air pressure from a compressor to the locking port, pull the pushrod out a bit and then it will slide back in.  The clutches need to have the pushrod move out a tad before they will release.   Put a strap around the whole cannister before you undo the clamp - there is a spring in there, not a 600 lb spring like on the spring brakes, but a fairly decent spring all the same.   Have fun, let us know how you make out.

Brian

[Cecil The Diesel]

So like I ratcheting tie down strap? Very thankful for the info.