MC7 will not air up (brake valve problem I think)

[oldmansax]

think this problem has been addressed once but I can't seem to find it.

When I start the bus cold, it builds air pressure to about 30# at idle and will not build any more. If I release the parking brakes, it will build up normally to 120#. Once I apply and release the brakes a couple of times, the problem goes away until I shut down & the bus cools off. I did not have this problem until it started getting cooler. It's around 40F here at night. It has the original style brakes (DDC3? DDEC3?) that came on MC7s. I notice a very small air leak from the exhaust on the big relay valve just ahead of and a little above the drive axle. I am assuming that relay valve is failing. Everything is working fine other than this problem. If I have to replace the valve, is it a standard part & can get from a truck place or do I need to order it from Luke or MCI?

Thanks,  TOM


ACT II- I got back to the shop & checked this again. When I start up and the air pressure gets to about 30#, air is leaking big time from the exhaust of the big relay valve in front of the drive axle. As soon as I push the parking brake release button in, air leak stops & air pressure starts to build, although, the bus will not move because the parking brakes are still holding. Once air pressure is up, I can apply the service brakes a couple of times & the brakes will release & everything is fine.   

[Stan]

Should be available at the truck parts store. Take the old one with you for an exact match. The bolts will be rusty and tough to get off. If you break the mounting ears on the valve there is no core credit and that is usually more than the exchange valve.

[Sammy]

Sounds like a bad park brake diaphragm in the DD3 brake chamber.
Air is supplied from the park brake tank to the park brake diaphragms when the park brake is "ON".
You can clamp off the lines one at a time at the brake chamber, to identify the bad diaphragm.
Chock the wheels so the bus won't roll, support the body real good before going under the coach.
Safety first. Take it to a pro if you're not sure how to do it safely.
Good luck. 

[buswarrior]

Yes, spend more time diagnosing the problem before you waste time and money changing the wrong parts!

You have a control valve and the foot brake valve up front, an inversion valve on the top of the differential, the relay valve, two diaphragms in each of two DD3 brake chambers, a pressure regulated feed from the park tank, and the full tank pressure of the service tank all interconnected.

If for whatever reason the internals of any of these is sticking, cracked, dirty, sitting wrong, or just plain old worn out, it often displays as leaks out the exhausts of the relay valve or the brake valve up front.

drain the wet tank, and then try draining in turn the parking tank and the service tank to identify the offending circuit.

Let us know!

happy coaching!
buswarrior

[Tony LEE]

From the MCI web site


Air is leaking out of the R-14 relay valve located above the drive axle (On ABS equipped coaches, air leaks out of the modulator valve.)

    * In the DD3 brake chamber, the park brake diaphragm separates the park brake section and the service brake section. If this diaphragm develops a hole in it, air will leak into the service brake section. When the service brakes are not applied, this section is vented to the exhaust port on the relay valve (With ABS, the exhaust port on the modulator valve.) Any air entering the service brake chamber will be exhausted out immediately thus causing the illusion of a faulty relay valve (With ABS, the modulator valve.)
    * To troubleshoot this situation, remove the service brake airline from one of the DD3 brake chambers. (This is the middle hose on the chamber) If air is leaking out of the service brake port, that is the chamber with the leaking park brake diaphragm. If air is still leaking out of the relay valve and the hose in your hand, then it is the chamber on the opposite side of the axle (On coaches with ABS, air leaking out of the RH modulator valve would mean the leaking park brake diaphragm will be in the RH DD3 brake chamber, LH modulator valve would point to a leak in the LH side.)

[Busted Knuckle]

From the MCI web site


Air is leaking out of the R-14 relay valve located above the drive axle (On ABS equipped coaches, air leaks out of the modulator valve.)

    * In the DD3 brake chamber, the park brake diaphragm separates the park brake section and the service brake section. If this diaphragm develops a hole in it, air will leak into the service brake section. When the service brakes are not applied, this section is vented to the exhaust port on the relay valve (With ABS, the exhaust port on the modulator valve.) Any air entering the service brake chamber will be exhausted out immediately thus causing the illusion of a faulty relay valve (With ABS, the modulator valve.)
    * To troubleshoot this situation, remove the service brake airline from one of the DD3 brake chambers. (This is the middle hose on the chamber) If air is leaking out of the service brake port, that is the chamber with the leaking park brake diaphragm. If air is still leaking out of the relay valve and the hose in your hand, then it is the chamber on the opposite side of the axle (On coaches with ABS, air leaking out of the RH modulator valve would mean the leaking park brake diaphragm will be in the RH DD3 brake chamber, LH modulator valve would point to a leak in the LH side.)

Tonylee,
that is good "BOOK SENSE" but in all actuality Sammy's advice works easier & faster! It's old school, but it workz----------!
  BK 

[Tony LEE]

Yes, quicker perhaps, but the time taken to undo a fitting is insignificant and once it is off, there is no doubt of the results. Squashing a braided hose with vicegrip pliers is not going to improve it much, especially hoses as old as the ones on most of our buses.

Main reason I quoted the entire passage was to emphasise that the method of changing components that seem to be faulty is not always all that logical and can waste a lot of money. The passage also fully explained why Sammy's suggestion was spot on and that helps all of us understand how the system works for next time.

As another example, I recently had trouble with the bus airing up - the red pressure went up reasonably fast but the white pressure lagged far behind.  I had all sorts of suggestions to change components - from the compressor, to governor to check valves - luckily none of which I did. Turned out to be a $25 brake protection relay that was sticking and not opening at 65psi as it should, that was quite obvious once I worked out how the system operated.

[oldmansax]

Thanks to everybody for the help thus far.

I thought I would get to look at my bus today but I had several service calls. Tomorrow is out also but maybe Thursday I will get to try out further diagnose the problem.

I will post the results.

TOM

[oldmansax]

I found the problem. Parking brake diaphragm on passenger side. I ended up removing the service brake hose just to make sure. Fortunately, I can still get under my bus even if the air bags are down as long as I park on a level surface. It was kinda tight this morning because the passenger side wheels were in a slight depression but I still made it.

I am going to replace both diaphragms when I do the job. That will save me more trouble later.

Thanks for all the help!

TOM

[FloridaCliff]

Tom,

I may be stating something you already know, But its easy to replace them without removing the cans.

No need to remove any of the brakes lines.  The help keep the can off the ground.

The service brake diaphram needs to be pushed in at the center or the cover is a little hard to get back on.

The park is almost too easy.

Best of luck

Cliff

[JackConrad]

    I just went through this on our MC-8.  The DD3 diaphragms are different than the standard diaphragms (and much more expensive!).  Only place I could find them was MCI Parts.  Service diaphragm was approx. $130.00 and Aux. (parking) diaphragm was approx $30.00.  Jack

[Busted Knuckle]

Jack did you try JD @ C & J? he usually has that stuff! He may not have the GMC or NJT stuff Luke has, but they got most of what you'd need for a typical MCI! FWIW   BK 

[JackConrad]

No I didn't.  I tried my local big truck parts place.  They quoted about the same price plus freight with 3-4 week back order.  MCI was in stock and free freight.  Jack

[Stan]

$160.00 would be a reasonable price. About 20 years ago I paid about $150.00 at a large brake shop supply in Phoenix. Of course, every smartass told me that diaphragms were only $3.00 each and that I got took. IIRC, front brake diaphragms were about $3.00 then.