Tracking down more air leaks

Cary and Don · August 29, 2009, 08:37:32 PM

We are on the war path now. We have found two more little leaks. The question, one is on the parking brake pull valve. Is there any reason why air would escape out the bottom of the parking pull valve during braking other than the seals in the valve being bad? The last leak is the pressure control valve on the auxilalry tank, but it is just toast. Need a new one. The bus is leaking down to 65lbs over night and then holds. It used to hold at about 80lbs over night. Two leaks to go.

Don and Cary
GMC4107
Neoplan AN340

MattC · August 30, 2009, 07:06:07 AM

And here I was thinking it was no biggie that the tanks bled out overnight. Oh, boy, guess more things to do.

JohnEd · August 30, 2009, 10:49:45 AM

D&C,

Doesn't theAux tank only start to air up after the supply tank reaches a certain level? You may have located your last and only leak.

Figure the odds on that being true.

Good luck,

John

Cary and Don · August 30, 2009, 03:55:55 PM

Our only concern is, when applying the brakes, air blows a little out the bottom of the park push pull valve. We tried changing the o-rings in it and the thing still blows a little air on applying the brakes. Maybe that is normal, but it doesn't seem right.

Don and Cary
GMC4107
Neoplan AN340

wildbob24 · August 30, 2009, 07:04:40 PM

D&C

That is not normal. Sounds like your problem is in the double check valve. It's mounted next to the rear air tank and plumbed between the park control valve and relay valve on it's supply side and the inversion valve on it's delivery side. If it's leaking internally, it may allow air to flow into the control valve when you apply the service brakes.

It can be bench tested with shop air. The valve is serviceable but it might be easier to just replace it, if it proves to be defective.

Cary and Don · August 30, 2009, 08:23:06 PM

I don't see that valve on any of my diagrams. According to the manual, the only check valve is on the supply side of the rear tank. When it gets light out there tomorrow, I will go hunting for it. We're getting really tired of this game.

Don and Cary
GMC4107
Neoplan AN340

wildbob24 · August 30, 2009, 11:08:07 PM

Yes, you are correct. I was thinking 4905(which is what I now have). I just realized that you are working on a 4107, which does not have that double check valve.

My 4107 experience is limited(I owned one for about 6 months), but the diagram shows the exhaust port of the park control valve plumbed into the delivery line of the foot valve, apparently so that the air in the DD3 lock port exhausts through the foot valve when you apply the parking brake.

When you step on the brake, you are sending air pressure back to the park valve exhaust port which would make me suspect that you still have leaky o-rings in the park valve.

bevans6 · August 31, 2009, 04:50:38 AM

On my bus, MCI, the service brakes, when applied, supply air to the lower exhaust port on the parking brake valve. That chamber is sealed with o-rings to the bottom. If air is leaking out, audibly, when the service brakes are applied, the o-ring seals or the port connection is leaking. this is incorrect, and technically a DOT failure that should take the bus out of service (no audible leaks allowed). Depending on how much of a leak it is, you might fail the 3 PSI leak on full brake application for 1 minute test. But if you have installed a proper repair kit, and it's still leaking, then get another valve and replace it. My manual shows the O-rings as .625" and .627", so actually slightly different sizes.

Bottom line, nothing is allowed to leak on a service brake application, nothing at all. My manual, obviously not for your bus, but - shows a quick release valve also near the parking brake valve and the foot valve, that is another possibility for an audible leak on service brake application.

Hope this helps a bit, anyway.

Brian

busshawg · August 31, 2009, 01:13:25 PM

Not certain . but doesn't air come back through the parking brake valve upon application if you have a faulty air pod, or diaphram in a brake pod?? Could be wrong but something seems to trigger a memory, maybe BW will jump in here.

I also had to replace my parking brake valve but the problem was when this valve was pulled to apply the parking brake. I tried replacing the o rings in the valve, but they are not 0-rings, they have a flat side on the from factory. I ended up just replacing teh valve form a truck supply store. About the third of the cost that MCI wanted.

bevans6 · August 31, 2009, 04:16:47 PM

If the parking brake valve is in the OFF position (in) for normal driving, there is full tank pressure fed to it, and from there to the DD3's lock port, said pressure used to keep the locking ring disengaged. If there was a problem with the service diaphram in the DD3, the DD-3 might leak all the time the PB valve was off. the only time a problem with the DD3 could send air to the PB valve is PB valve is ON, parking brake applied. then there should be no air pressure at the locking port, so that the locking ring is free to engage. At that time, there would be tank pressure at the parking brake port on the DD3, none at the service port, and one at the locking port. If there was a leak in the parking brake diaphram, the leak would feed back through the service port up to the exhaust port of the PB valve. Which still shouldn't leak, i don't think.

The OP has a leak during service brake applications, which means the PB is off, and there is pressure in the upper half of the PB valve all the time, and pressure in the lower half only on service brake applications.

I think, anyway. More than willing to be wrong here. Just studying the logic of the flow...

Brian

Cary and Don · August 31, 2009, 07:27:18 PM

It doesn't leak with the parking brake set or off. It only leaks when brakes are applied. If the seals are not o-rings, but a flat sided seal, that could be the problem. It had o-rings in it and we replaced them with new o-rings. Didn't help a bit. The diaphram looked fine. The spring is fine.

Don and Cary
GMC4107
Neoplan AN340

buswarrior · August 31, 2009, 07:36:49 PM

My head hurts...

Ok, when you put your foot on the brake pedal, this air is coming out of the control valve that is beside the driver, the one you use to apply and release the parking brake?

And this happens while you are rolling down the road, pulling up to a stop sign, waiting at a light, for instance?

Did we clarify which kind of parking brake system this is? Spring or DD3?

And the diagnostic procedures used to verify the various parts listed as "good" were?

There are several languages being spoken, we'll get it down to one!

happy coaching!
buswarrior

Cary and Don · August 31, 2009, 07:55:04 PM

They are DD3 chambers. This isn't a lot of air, but air all the same. If you release the parking brake, no air. Set the parking brake, no air. Release the parking brake and apply the brakes with the pedal, air leaks out the bottom of the parking control knob. Air blows as long as the brake pedal is down. This isn't enough air to make the gauge to drop dramatically, but it will drop maybe a pound in a couple minutes if you hold the brake pedal half way down. We have soaped all the chambers and the relay valve, inversion valve, and the lines. Can't find any leaks back there.

Don and Cary
GMC4107
Neoplan AN340

buswarrior · August 31, 2009, 08:00:19 PM

Leaking seal in the parking control valve.

It shares its exhaust plumbing with the service brake circuit.

Applying the service brakes pressurizes the exhaust of the parking control valve, and appears to be forcing air past the seal.

Rebuild, or my usual choice, replace.

happy coaching!
buswarrior

Cary and Don · August 31, 2009, 08:06:01 PM

Oh, we were hoping it would be that simple. Now if we can just find one of those pressure control valves that go on the axillary air tank, we will be in good shape. Anybody have one?

Don and Cary
GMC4107
Neoplan AN340