Rear Brakes will not engage

[miles2go]

Thanks Brian. That is encouraging... which is really needed!

Andrew.

[zubzub]

Forgive my ignorance in this (which I am displaying handsomely today) but if blocking the exhaust port of the relay allows operation of both pushrods would that not eliminate a blown diaphragm? Especially as there is no movement of either pushrod  when exhaust port is left open, which would seem to dismiss a smaller leak in a diaphragm.

[miles2go]

That does make sense. I am clueless but learning fast due to all the great help from the board. That is why I was asking about whether blocking off that port to make the rear brakes work would damage anything else. I am trying to find out what an inversion valve does and why that port is there. If a plug fell out, or if it is an exhaust port... but I will try all avenues of diagnosis to make sure, if they have been suggested, thats how I learn.

Thanks,
Andrew.

[Len Silva]

There is not a lot about DD3 brakes here but the Bendix Air brake manual is a must have for busnuts.

http://www.wsafc.org/WSFMA/Shared%20Documents1/Bendix%20Air%20Brake%20Handbook.pdf

Here is a little info on the DD3 chambers.
http://www.bendixvrc.com/itemDisplay.asp?documentID=2393

[Tenor]

I just spent a whole lot of time working on the relay valve that you have in the picture.  To my embarrassment, I re-installed in wrong.  On My MCI7, (as you look at your picture), 1 line goes to each DD3 chamber, one port goes to a pressure limiting valve (which then goes to the tag brakes -yours might not be plumbed the same way), one line is the supply line.  On the top half, one line goes to the brake treadle valve.  There is no way the relay valve will work if the plug is missing.  Air is only supposed to be evacuated through the rubber exhaust port.  Even if you replace this valve with a new one, you are most likely still going to need to plug that port.  I have seen weird things end up missing due to neglect.  I would spend good time following your air diagram to make sure which lines go where, and then plug the hole and CAREFULLY test the bus. 

Good luck!

Glenn

[Len Silva]

You might also take a close look at the front brakes.  They may have been doing all the braking for a long time.

[wildbob24]

Andrew,

The valve in the lower part of your photo is the relay valve. The component in the upper part of your picture is not the inversion valve. The inversion valve is part of the parking/emergency brake system and will have 5 air lines running to and from it. It is not in play when you're operating the service brakes.

That thing looks like a check valve. It's a little hard to tell but that line looping to the right looks like it runs from that valve to the signal port of the relay valve. Is there another line running into the top of that smaller valve? I don't recall there being anything between the application (foot) valve and the relay and don't know why it would be there. The diagram in the book doesn't show any component in that line.

I'll try to get under my Buffalo this afternoon and see if there is one on there. Maybe that will tell us something.

Bob

[Lin]

I definitely was looking at the relay valve that takes up the bulk of the picture.  I am not sure what the other one is.  My 5a has a quick release valve that is somewhat similar to what I can see of the part on the top of the picture.  This allows exhausting of air from the brake chamber when the foot brake is released. 

[miles2go]

Thanks everyone, still in doubt though. Here is a picture of the component that the name of is still not verified, last pic has a plug I installed (but did not test for lack of another person to push brake pedal while I watch for movement).

It sits above the relay valve and below the rear air tank on the rear bulkhead of a GMC 4905. There seems to be a supply line to it from above, maybe from the main air tank (it is hard to tell), a line going to the relay valve (possibly the signal port?), and a line going to a valve mounted off of the rear air tank that feeds the DD3 cans.

[miles2go]

Maybe a better picture?

[miles2go]

This is the valve? on the rear air tank that one of the lines from the mystery component goes to. The rubber hoses on the ends to the right side of the pic go to the rear DD3 brake cans. The copper line next to them (next to last) is from the mystery component (that may be a quick release valve?). If so, did I just plug the quick release port? cant tell yet until I have someone to press the brake pedal.

[miles2go]

Plugged... for now. Until tested or feedback...

[buswarrior]

That last picture shows the inversion valve, the one to the left with the rubber exhaust cover visible.

(edit: the picture above the "plugged" picture...)

Perhaps that is a regulator mounted in one of the inversion valve ports? As well as a bunch of "T" fittings.

I suspect some creative assembly by some previous authority, which you might want to scrap and return it to proper configuration.

And it ALL really needs to be replaced. Valves are cheap, they are consumables, and yours look like they have been through a war or two. The lines attached to them will be shot as well.

Cut them and carefully tag and catalog so you can get replacements made and re-installed in proper order. Better still, get another busnut to give you a hand!

You need to trace where these lines go, "maybe" and guessing don't help us from this distance to trouble shoot very well.

happy coaching!
buswarrior

[Len Silva]

I don't know for sure about your 4905, but I do know that the manuals I had for my 4104 were extremely comprehensive.  I think that if you have the manuals, all will become very clear and it become very easy to figure out what you have and what you should have.

[Tenor]

Now that we have different pictures, please disregard my previous post.  I thought that a plug was missing from the left side of your relay valve.  I've been through my MCI manual, and I don't find a corresponding part.  Your GMC manual should have a very clean picture of it, unless you don't have that manua...

Good Luck!
Glenn