Air Brake Leak Diagnostic Procedure

[jap42]

So I have a constant small leak of air from the relay valve on my 89 MCI bus. I am looking at the plumbing and reading and watching every detail of how the system works. As I understand it, if the parking brake is set and the service brakes are released then there should not be air anywhere to leak in the brake chambers to the vent, so the leak would have to be from the supply line meaning the valve itself is leaking, right? Or is there another path to the air.

Also, when I pull up on the Parking Brake to apply it shouldn't I hear air being released? I dont hear pressure release until I let go of the service brake. The parking brake releases, but I am not totally sure that it is holding. So far everywhere I have parked has been flat and it would not roll anyway.

[buswarrior]

You have DD3 brake system.

Forget everything you thought you knew, because that would have been learned from a spring brake system.

Your rear brake chambers both stop and park with air. Then some trick locking rollers keep the brakes on, if the air leaks away.

The double diaphragm in the rear chamber will leak to the relay valve, if it has failed.

So, it might be your relay valve, leaking in its own right, or one of the dd3 chambers leaking through from the parking portion.

A process of trouble shooting will be added shortly, by someone, or me, I am short on time this AM.

Happy coaching!
Buswarrior

[jap42]

Ok, that helps a lot. I was able to pull up a disgnostic page from bendix on it. So I will start by blocking the tires and removing the parking brake. If it's still leaking then it's the valve, if not then it's the chambers. Sounds like it's more likely the chambers.

Can they be rebuilt, or should I just replace them? Where can I get them. I have been looking online and can't find them.

Has anyone ever, or would it make sense to change this to a spring brake system?

[lostagain]

If the housings aren't too rusted, you can easily rebuild the DD3 chambers with an Oring kit and new diaphragms. Available from from US Coach, and other places. And then keep them greased.

JC

[Jim Blackwood]

As I understand it, you are not supposed to apply the parking brakes while standing on the brake pedal.

Jim

[jap42]

So I released the parking brake and service brakes after blocking the tires. The same air leak is there, I had my wife apply the service brakes and the same leak was there. I went under and while feeling around I have found that the air seems to be coming from around the vent, rather then actually out of the vent. I blocked the vent off with a socket and the air sound did not change at all.

So, are the valves rebuildable or should I just replace it. Is it any different because of the DD3 system? Looks like a regular relay valve.

I tried looking up US Coach but could not find it. So far I can not find a dedicated online retailer for coach parts. No one local ether.

After reviewing the setup of the DD3 system, you can have your foot on the brake. The parking brake locking ring locks into place if the cylinder looses air, to unlock it you just need to be able to apply more air then when it was set. I think 44 PSI was the number, maybe 64. It normally does not require more then 20-30 lbs to hold a coach still while you apply the brake. As long as you dont have the peddle mashed all the way to the floor when you apply you will be able to unlock it.

[bobofthenorth]

US Coach 888-262-2434

[peterbylt]

I replaced the relay valve on our 89 MCI 96A3.
Same symptom as yours leaking near the exhaust.
First of many parts that I Bought from Luke at US coach.
Pain to get to, up in front of the differential.



Peter

[jap42]

Our local Napa had a reman in stock, so I got that. I'm going to back onto some blocking to get some extra room under the bus. Nothing looks too horrible under there. Hopefully the fittings don't fight too much.

[chessie4905]

Be careful not to get the hoses mixed. On mine, the mounting bolts go through the bulkhead. Had wife hold a wrench on bolts while I removed the nuts underneath. I have a pit, so it was much easier.

[jap42]

So I got lines loosened and could not get the rubber hoses to the forward axel to come off the fitting. I retightened everything and pressure checked it again. Still leaking, deffinatly around the vent. There's just no room to do anything under there. I wish this had engine trouble. I can climb in that bay and close the doors.

Going to get a quote from a few local garages.

One of my air bags was leaking air around the top where it meets the top plate when I supplied my shop air. (75psi) it went away when the pressure reached 120 and seems fine now. Is that an issue or are the bags 2 peice and just not seated yet?

[chessie4905]

Try removing the rubber hoses at the brake chamber end first and take valve off with hoses still attached to it. I replaced the hoses on mine while I was at it. They are common, cheap and available at any truck parts dealer. Just match length and fittings.

[Jim Eh.]

Ditto. If they are anywhere near original, just replace the complete hose assembly with a ready made unit. There are reuseable fittings available for rubber hose but they cost almost the same as a ready made hose and if you use them, you still have old hose.

[jap42]

The forward axle is fairly new rubber compression hose. I was literally hanging off the hose with the compression nut totally removed and they would not budge. I will have to cut them and replace them. I have a fitting press so I can make hoses. But if its cheap enough I will just have the garage do it so I can focus on the interior.

[chessie4905]

I cut 4 of mine because replacing them. Not worth screwing around trying to take them off carefully to put in trash can.
Example:https://www.napaonline.com/en/p/GAT27267

Google air brake chamber hoses. Many sources, including Amazon. Just match length and thread type.