No air pressure!

[Brad_SwiftFur]

Now you're getting into the heart of the matter and doing a good diagnosis. You've started a good process of elimination, finding out "This is good, that works properly", as well as following the stream, so to speak, until you find out where it stops.


This will probably end up one of those difficult to find, (relatively) easy to fix things.

[Wornout Wrench]

Originally Posted by Mtrdrms

So the tank closest to the air dryer has 4 lines. I think this is the wet tank? One of the lines has this shraeder valve on it.

Yes, that should be your wet tank.

If you go back to my post where I had the service manual up, open it up and go to page 8. ( I can't seem to figure out how to post the picture )


So what you are seeing in your picture will be items
p), wet tank
M), the schrader valve
O), the safety valve
The small plastic line should be the one going to item J), your air governor.

They just have them all running out of the same Tee.

So that is 1 of the 4 lines

Looking back at the diagram again, you have your air drier in line, the line out going to the other air tanks and 1 more line going to item K) pressure protection valve. In reality I would expect K to be mounted on the tank and the line coming off of it. Your PPV (Pressure Protection Valve) runs all the auxiliary air on the bus so this is where your air bag air will come from. The valve is closed until a set pressure is reached, not sure what it would be on a bus but say 65-70 PSI. The line out should not be smaller then the in and out lines, but bigger then the line out to the governor.

So if you can figure out which line is in from the air drier, the other big line should be the line out to the other tanks.

[Wornout Wrench]

Originally Posted by Mtrdrms

The line from the compressor appears to go to the bottom of the air dryer

Yes, there will be 2 big lines plus the small plastic hose coming from the air governor

[Wornout Wrench]

Originally Posted by Mtrdrms

Ok, so I went to the front of the bus and disconnected an inlet and air was moving but I could easily block it with my finger. It wasn't building any pressure.

Looking toward the rear, from under the front, there is a whole host of lines and things:

So the hose that you took off, was it the one coming from the wet tank?

If that was the line that you took off and it would not build pressure, your very close to finding a fix

[Brad_SwiftFur]

What eventually ends up happening, at least for me, is actually crawling around under the vehicle and listening for air leaks. I use my hearing to zone in on it and feel around the source for moving air. I've found more than one failed brake diaphragm, leaking air line, and so forth this way.

[Wornout Wrench]

Originally Posted by Brad_SwiftFur

What eventually ends up happening, at least for me, is actually crawling around under the vehicle and listening for air leaks. I use my hearing to zone in on it and feel around the source for moving air. I've found more than one failed brake diaphragm, leaking air line, and so forth this way.

You nailed it Brad.
There is no easy way, and if the hose is bundled in with a bunch of other stuff, it can be a real ***** (insert favorite saying here) to find.

Hands, ears and some soapy water.

And patience, lots of patience.

[Booyah45828]

Hey! Now you're doing some diagnostics!

The line from the drier to the wet tank. At the wet tank, check that line in the same manner you've been doing.

What do you get?

A) If you have air pressure there, we'll test the wet tank and it's components next.

B) If you don't have pressure there, then the line between the drier and the wet tank is leaking.

[Mtrdrms]

Ok, thanks guys. This is helpful. I'm learning here.

In this picture you can see something that appears to be a check valve. There are several throughout the bus. They are all leaking small amounts of air from the diaphragm on the bottom. Is this normal?

[Mtrdrms]

Originally Posted by Booyah45828

Hey! Now you're doing some diagnostics!

The line from the drier to the wet tank. At the wet tank, check that line in the same manner you've been doing.

What do you get?

A) If you have air pressure there, we'll test the wet tank and it's components next.

B) If you don't have pressure there, then the line between the drier and the wet tank is leaking.

Ok, I'm going to do that now...

[Wornout Wrench]

Originally Posted by Mtrdrms

Ok, thanks guys. This is helpful. I'm learning here.

In this picture you can see something that appears to be a check valve. There are several throughout the bus. They are all leaking small amounts of air from the diaphragm on the bottom. Is this normal?

That's not a check valve and should not be leaking

[Booyah45828]

Your picture shows what looks to be an abs dump valve. There should be a hose entering, a hose leaving, and some wires to it.

There should be one of those near each wheel.

The rubber part isn't a check valve, but a dust cover over the exhaust port to keep dirt out.

That's really odd that that is leaking because there shouldn't be air there unless you're applying the brake pedal.

How much air is leaking out of them?

[Mtrdrms]

Ok, I disconnected the line from air dryer at the wet tank. It easily makes 100 psi.

In addition to that line on the wet tank there are 2 red lines that join together, another 1/2" black line on the other side of the tank and the small line with the valve in the picture from a previous post.

I disconnected the OTHER black line on the tank and checked pressure there with the bus running. It would not read on my guage.

So, the air that is leaving through the RED lines leads to the leak....I think.

[Mtrdrms]

Originally Posted by Booyah45828

Your picture shows what looks to be an abs dump valve. There should be a hose entering, a hose leaving, and some wires to it.

There should be one of those near each wheel.

The rubber part isn't a check valve, but a dust cover over the exhaust port to keep dirt out.

That's really odd that that is leaking because there shouldn't be air there unless you're applying the brake pedal.

How much air is leaking out of them?

Not much but I can certainly feel it. From each one. Could this be a brake pedal issue?

[Brad_SwiftFur]

Originally Posted by Mtrdrms

Ok, thanks guys. This is helpful. I'm learning here.

In this picture you can see something that appears to be a check valve. There are several throughout the bus. They are all leaking small amounts of air from the diaphragm on the bottom. Is this normal?

Originally Posted by Wornout Wrench

That's not a check valve and should not be leaking

It's a little hard for me to tell from that pic - do these feed the brakes? Many air brake systems have these valves near the axles (and many road tractors have them for the trailer lines) - (assuming it's what I think it is) what they do is allow the air to supply the brakes when you apply them, but when you release the air, they allow the air to quickly vent for faster brake release. They should only allow air to escape when you release the brake pedal - the rest of the time, when your foot isn't on the brake, there shouldn't be any air pressure in the service brake system.

[Brad_SwiftFur]

Originally Posted by Mtrdrms

Ok, I disconnected the line from air dryer at the wet tank. It easily makes 100 psi.

In addition to that line on the wet tank there are 2 red lines that join together, another 1/2" black line on the other side of the tank and the small line with the valve in the picture from a previous post.

I disconnected the OTHER black line on the tank and checked pressure there with the bus running. It would not read on my guage.

So, the air that is leaving through the RED lines leads to the leak....I think.

Red lines usually indicate brakes - parking and/or service brakes. From the air tank, especially if it's a sizable red line, probably a supply for the whole system. This will get full pressure at all times, No Matter What. The idea is that if you step on the brake, you need to stop *NOW* using whatever air pressure is available. I won't get into the 60 PSI and parking brake release right now because we're diagnosing an air loss issue.

Originally Posted by Mtrdrms

Not much but I can certainly feel it. From each one. Could this be a brake pedal issue?

Feeling air from *ALL* of these isn't normal and I find it hard to believe all of them failed at once. These are not known to fail that much at all, so it leads me to think there's air being supplied to them when it shouldn't be.


Maybe you do have a brake pedal issue? It's not really much more than a foot controlled pressure regulator.


At this point I'd try removing the red line you mentioned and capping off connection and see what happens.

[Wornout Wrench]

Originally Posted by Brad_SwiftFur

It's a little hard for me to tell from that pic - do these feed the brakes? Many air brake systems have these valves near the axles (and many road tractors have them for the trailer lines) - (assuming it's what I think it is) what they do is allow the air to supply the brakes when you apply them, but when you release the air, they allow the air to quickly vent for faster brake release. They should only allow air to escape when you release the brake pedal - the rest of the time, when your foot isn't on the brake, there shouldn't be any air pressure in the service brake system.

Brad, you are thinking of Quick Release valves, this isn't a QR.
I believe Booyah is on the right track with it being one of the ABS modulator valves.

But yes, it should only have air to it when the service brakes are applied.

It is a glorious day on Vancouver Island.
I'm heading off for a 50Km bike ride.
I'll check in when I get back.

You guys are on the right track, the process of elimination will get you there.

I would worry about the big leak first and deal with the secondaries later once you get air up.

[Brad_SwiftFur]

Originally Posted by Wornout Wrench

Brad, you are thinking of Quick Release valves, this isn't a QR.
I believe Booyah is on the right track with it being one of the ABS modulator valves.
.....
But yes, it should only have air to it when the service brakes are applied.

I think some ABS modulators are integrated with the Quick Release valves?


At any rate, it should only get air when you step on the brake. Or in the case of some of these newer vehicles, if the anti-collision computer applies the brake. If it's getting air when it's not supposed to, I'm suspecting a leaking brake pedal valve.

[Mtrdrms]

Enjoy your ride! 50k! Wow

[Mtrdrms]

Originally Posted by Brad_SwiftFur

I think some ABS modulators are integrated with the Quick Release valves?


At any rate, it should only get air when you step on the brake. Or in the case of some of these newer vehicles, if the anti-collision computer applies the brake. If it's getting air when it's not supposed to, I'm suspecting a leaking brake pedal valve.

Ok, I just confirmed that there are 4, they each go to a brake booster and they are all leaking.

[Mtrdrms]

Is the brake pedal valve right under the brake pedal?