DD3 brake chambers

solodon · July 07, 2014, 12:11:10 PM

Hi, I've read the archives on this. I started with air coming out of the Relay valve, and when looking here found that is more likely a problem with the brake chamber. I checked today and there is air escaping from the relay valve exhaust on the bottom whether the parking brake is activated or not. Seems this could mean the relay valve but could also mean the chamber. I've done some looking online to see if I can find diaphrams, and part numbers, and only found aux. Going to park it, raise it, remove the wheels and tires and air it up again so it's easier to evaluate. Hoping it's the chambers, as it's easier to work on. It's a '79 MC9 and may be original. This all happened out of the blue. Any suggestions would be appreciated.

TIA, Don

bevans6 · July 07, 2014, 12:36:58 PM

http://rebuildersenterprises.com/ is who I dealt with for new DD3 chambers. They have all the parts and can supply new and rebuilt units. Luke probably can too, at US Coach Repair 856-767-4848. Pinch off or remove the main line hose to each chamber in turn to see if the leaking at the relay valve stops. Sources of leakage with the bus aired up, the brakes not applied and the parking brake off are the relay valve itself, or a chamber is leaking from the pressure at the locking port leaking into the service chamber and out the exhaust port on the relay valve. In that state there is air pressure to the relay valve from the dry tank and there is pressure at the locking port (closest to the push rod on the DD3) so that's where the air can come from. If there is leaking at the R-8 valve when the parking brake is on you can suspect a failed diaphragm in the chamber.

Brian

Cary and Don · July 07, 2014, 02:50:45 PM

Before just buying new chambers, it would be good to check and see if they just need diaphragms. They are cheap and easy to change. We changed ours without even removing the chamber from the bus. Chambers are a good chunk of money.

Don and Cary

solodon · July 08, 2014, 02:55:41 PM

Thanks for the two responses so far. Once I get it up and the wheels off I'll do the testing suggested. My hopes and plan is to replace only what needs replacing. I'd read here on the board that just replacing the diaphrams seemed all that might be needed and I feel comfortable with that type of task. Once I know for sure what is needed I'll go from there. I'd much rather rebuild the chambers than change the relay valve. Too many chances for leakage afterward with a valve change. I've got a couple of other things to check while it's in the air so might as well get it all at once. Again thanks to the information, and have a blessed day.

Don

Jim Eh. · July 08, 2014, 07:23:35 PM

Why not remove one end of the air lines that run from the relay valve to the service side of the chambers and find out if air is escaping from the chamber?

solodon · July 13, 2014, 06:32:11 PM

OK, finally got under and removed lines. No air from the service or parking ports on the chambers, with the parking brake set. Air at the locking port as expected. Air coming from relay valve to service port line. When lines are blocked, air escapes from the relay valve exhaust port. IF...I'm understanding this correctly, the relay valve is leaking and air is trying to go to the service port. With no leakage in that line the air is forced out the exhaust port of the relay valve. This would mean I have to rebuild the R-8 relay valve? The manual I'm using says R-12 but it's for a '89 and mine is a '79. Am I understanding this correctly???

Jim Eh. · July 13, 2014, 08:31:55 PM

If:
the delivery ports are plugged/blocked on the relay valve
and you are getting air leakage out the exhaust port with steady application pressure (or even with zero pressure applied)
then
yes, rebuild or replace the valve.

The crack pressure of the R-8 is about 3 psi and the R-12 is about 4 to 5.5 depending on exact version. They are all now "coreless" valves which means there is no core charge so if you wish to change to an R-12.

solodon · July 14, 2014, 05:45:33 AM

I'm guessing that delivery ports is the same as service port. I've not blocked the service port at the valve as I've disconnected the line at the DD3 chamber. When there is air in the system, and the locking port has air in it, air escapes through the hoses that go to the chambers. When I put my thumb over the open end of the hose air starts to escape out the exhaust port on the bottom of the relay valve. I think we're both saying the same thing just with different words but want to be sure as I hate fixing what ain't broke. What does "cracking pressure" refer to? I'm thinking that it means the pressure needed to open something but unsure as to what that something is. I realize there is little difference between 3 psi, and 5 psi but does it make a difference in how the brakes react/work?

Don

chessie4905 · July 14, 2014, 06:39:09 AM

Here is the Bendix air brake handbook. Study this and copy or get a printed version. Keep it along with coach service manual. Invaluable for everyone to have. Oops... here it is:

http://www.suspensionspecialists.com/techinfo/Bendix_Brake_Handbook_2009.pdf

luvrbus · July 14, 2014, 07:23:31 AM

It's probably in the plumbing I thought that vintage of bus used a R-6 with a separate quick release valve and there is not need for anti compounding feature with DD3 brakes the R-12 is a common relay for spring brakes what model of R-12 do you have ? is it a double check or single check valve

solodon · July 14, 2014, 07:49:29 AM

My reference material is the manual I downloaded from the internet. I think I might have gotten it on this site. Anyway, it's for an '89. In the brake section it is showing the DD3 chamber and not a spring brake chamber. I've not removed the valve yet and have no idea as to single or double check valve. The plumbing looks the same as in the manual and don't know about quick release valves. Seems the more I learn the more I don't know. Sorry for my ignorance but you folks certainly do help with the cure.

luvrbus · July 14, 2014, 08:03:55 AM

I only saw the 1979 in your profile

chessie4905 · July 14, 2014, 02:55:42 PM

Also, this source: And corrected previous post with missing link.

http://www.mcicoach.com/service-support/serinfo/serinfo04E.htm

solodon · July 14, 2014, 04:35:35 PM

Chessie4905, thanks for the links. Looks like "interesting" reading. Important too when you do your own work. Thanks again.

solodon · July 15, 2014, 11:09:40 PM

Thanks to everyone for the information, and links. After reading, checking diagrams, and doing tests on the bus I determined the diaphrams are ok and the problem is the relay valve. THEN I called Luke and he had a nice gentleman named Bill call me back. Bill works out in the shop. After telling him all I'd done, and what I'd discovered about the air leakage he confirmed that it's the relay valve and I'll be calling back to buy one. Again thanks.