No air pressure!

[Mtrdrms]

Any suggestions on where to get an air compressor?

 

[Brad_SwiftFur]

At this point, I don't think replacing the compressor will solve the issue. You felt air moving. You're hearing it in the tank and somewhere else. The compressor is doing its job. How's the air dryer (if you have one)? Is a line disconnected somewhere? Since you're showing 0 air, perhaps the air line for the gauge is loose? Follow the sound of the hissing.


While the engine is running, open a tank drain valve - is there any air pressure?

 

[Mtrdrms]

I opened all the valves I could find. No pressure at all.
I found the dryer. Nothing looked loose or leaking. No hissing noises anywhere.

 

[Booyah45828]

You're clearly moving air if you can't hold your thumb over the compressor outlet. You've already thrown a governor at it only to find that it wasn't faulty. Are you sure you want to change a compressor at several hundred dollars in cost to find the same thing? Stop throwing parts at this, get your hands dirty, and do some diagnostics. Pull lines and see where you have air flow. Did you check for air flow after the drier?

 

[Mtrdrms]

Ok, I'm going to get a compression tester and verify the compressor is good

 

[CHEESE_WAGON]

Also, I was poking around underneath and found a box by the rear axle with 3 red lights on it.

I'm thinking that's going to be an ABS controller box, though I can't for the life of me figure why that wouldn't be closer to the treadle valve on a bus. Unless it has separate controllers for the steer brakes and the drive brakes. Or only has ABS on the rear axle (stupid).

 

[Sleddgracer]

I'm thinking that's going to be an ABS controller box, though I can't for the life of me figure why that wouldn't be closer to the treadle valve on a bus. Unless it has separate controllers for the steer brakes and the drive brakes. Or only has ABS on the rear axle (stupid).

I don't know the year of the bus in question, but my new from the factory, 88 GMC pickup had ABS on rears only - I don't know when ABS became standard on all four wheels - I thought it was crazy when I bought the truck and told the dealership what I thought

 

[CHEESE_WAGON]

I don't know the year of the bus in question, but my new from the factory, 88 GMC pickup had ABS on rears only - I don't know when ABS became standard on all four wheels - I thought it was crazy when I bought the truck and told the dealership what I thought

Yup, extremely stupid. And if you remember, Ford (and Dodge, IIRC) had a field day on that when they brought out their nice new 4-wheel ABS system.

Not to mention those systems were notoriously flaky. Controller failures, you name it. Most people eventually just unplugged the controller and disconnected the warning light.

Incidentally, GM made a number of brake system boo-boo's until they made 4-wheel hydraulic ABS standard around '92. I had two different cars with some sort of 'passive' ABS, meaning some sort of self-adjusting proportioning valve in the system somewhere. When they failed...

Stomp the brakes once, your left side locks up. Stomp them again, maybe your LF and RR lock up. Stomp them again, the fronts lock up. Both of these POS's nearly bankrupted me before I sent them where they belonged. Shame, both cars ran and drove great, I just couldn't trust the brakes.

Sadly, GM has not learned their lesson, and I have bought my last GM vehicle for a daily driver.

 

[Sleddgracer]

Yup, extremely stupid. And if you remember, Ford (and Dodge, IIRC) had a field day on that when they brought out their nice new 4-wheel ABS system.

Not to mention those systems were notoriously flaky. Controller failures, you name it. Most people eventually just unplugged the controller and disconnected the warning light.

Incidentally, GM made a number of brake system boo-boo's until they made 4-wheel hydraulic ABS standard around '92. I had two different cars with some sort of 'passive' ABS, meaning some sort of self-adjusting proportioning valve in the system somewhere. When they failed...

Stomp the brakes once, your left side locks up. Stomp them again, maybe your LF and RR lock up. Stomp them again, the fronts lock up. Both of these POS's nearly bankrupted me before I sent them where they belonged. Shame, both cars ran and drove great, I just couldn't trust the brakes.

Sadly, GM has not learned their lesson, and I have bought my last GM vehicle for a daily driver.

yeah, GM, Ford, and Dodge have all had their screwups - that 88 GM I had was a great one to drive - good mileage, comfortable, lots of power, arrive feeling fresh after a trip - sure beat the 87 dodge I had been driving before I got the GM - that dodge was a good ten years behind on their technology - I think Ford sales were still suffering the backlash from the ill conceived holes they cut in their frames in the early 80's when I bought the 88

 

[Mtrdrms]

Ugh, well the search continues. I can find no leaks. The compressor is making well over 150psi. I have 0 air pressure in the bus. Does anyone have any suggestions of what I can do now? I'm out of ideas.

 

[Sleddgracer]

Ugh, well the search continues. I can find no leaks. The compressor is making well over 150psi. I have 0 air pressure in the bus. Does anyone have any suggestions of what I can do now? I'm out of ideas.

all l can suggest is to follow the air lines and check all valves to be sure they are closed and not leaking

 

[HazMatt-SKO]

It's GOT to go somewhere! Presuming the gauge functions, there are no honked-off big snake-sounding hisses to be heard, is it possible the fitting that discharges air from the tank is blocked? Or possibly the line downstream from it. Or subsequent connections.
I'd disconnect from the tank, let it build some pressure, and (assuming quick-connects) attach a known open hose to it.
If it blows, which is good, reconnect the original, and repeat on down the line...

 

[CHEESE_WAGON]

I'd start thinking about a blockage, or perhaps a faulty gauge.

Have you checked the drier? Most systems have a drier on them that on many also serves as a filter. It's recommended they be changed fairly often (yearly with regular use). I've not heard of one stopping up, but if it hasn't been changed recently, you might check that. It's not common for these to stop up, otherwise I might have mentioned it earlier.

 

[Booyah45828]

Re-attach the hose and follow it back to the air drier and do the same thing. If it's building pressure there, reattach the hose to the drier and remove the drier outlet. If you're building pressure there reattach the hose to the drier and do the same test at the other end of the hose.

Did you do any of this?

Where did you see that you were making well over 150 psi?

This is truly simple.

If you have air out of the compressor, re-attach the line and follow it to the next component, and see if you have air coming flowing through that. You just keep following the lines until you find the leak.

 

[Mtrdrms]

I know what you're saying but man, there are a lot of lines. In addition to the compressor and governor which has 2 plastic lines (what does this thing even do and where do those lines go?)This thing has at least 3 tanks, each with 2-4 lines coming out of it, a dryer that looks like an octopus, all of the air actuators for the brakes, 4 air bags with their lines, lines that go up to the cockpit for the door, ebrake, guages...
Also, I think there are 2 A.C. condensers down there with a ton of lines and they're all routed with the air lines.
Now, if that wasnt enough, this thing is on the GROUND with FLAT air bags. AND it has underneath storage and lots of lines run above that compartment - inaccessible.

I'm telling you, this is a friggin pain.

 

[HazMatt-SKO]

I hear that! It sounds like a target-rich environment. But, all you can do is all you can do...
Isolate. Attack. Move on.

 

[Wornout Wrench]

I am not familiar with the air system on a school bus having never worked on one, but I have worked on air equipped vehicles for 40 plus years.

From what you have posted it looks like there is nothing wrong with your compressor. Don't even think about the governor yet, it does nothing until you get air pressure up.
The reason you are not seeing any air pressure on your gauges is because of where they are taking the pressure readings at. The pressure senders will be on the other tanks, should be 2 air pressure gauges for your tanks, A & B.

You have got a major leak somewhere between your compressor and the tanks with the senders on them.

So, you need to start looking at the lines from the compressor to air drier
Air drier to wet tank
Wet tank to the rest of the system.

There are check valves here and there through out the system, single checks and double checks. Your wet tank should not drain back through the compressor.

Things like the air horn, air ride seat, any air accessories on the bus and possibly the air ride should come off of the wet tank. Sometimes there are valves that will not allow the air to flow until it reaches a certain pressure, once again, not knowing the system I can't comment for sure.

The compressor doesn't make huge volumes of air so it doesn't take that big of a hole to let all the air out.

Having a helper will help here.

Start up the engine, let it warm up for a few minuets then rev it up, take it up to 2000 rpm. It will not hurt it, after about 10 seconds, while one of you is at the back of the bus, shut the engine down and see if you can hear any kind of a leak. It's going to take a lot of time but keep going and see if you can isolate the area where the leak is. You can keep the engine revved up for a bit longer to try to build more pressure and see what happens.

Silly question, you have made sure that all of your tank drains are closed?

If I had your bus in my shop I would basically rig up a remote air line into the tank and start looking for the leaks, that way no engine noise to cover up the air leakage.

On a side note. How is your hearing. I had one guy working for me that couldn't find an air leak that was so loud I heard it in my office 50 feet away. I went down one side of the truck, one of my apprentices went down the other and we both zeroed in on the hole in the hose. Another set of ears could be a big help.

I know it is a big pain in the butt to try to get to things, trust me, after 40 years of pulling wrenches, I know. But getting into it is the only way you will find it. Get some blocks and jacks and get it blocked up if that is what it takes. Don't go under if jacked up, with out blocking.

 

[Mtrdrms]

This is great information. Thank you!
None of the tank drains are open and my hearing is good...I think.
I like the idea if a remote fill for the system because the engine is so darn loud and if the leak is near the engine as you say, it would be impossible to hear.
I've got a friend coming by to help. I'll rport back with my findings.
Thanks again.
Andy

 

[CHEESE_WAGON]

I second Wornout Wrench. Probably not the best way to do it, but you could temporarily replace your drain cock valve with a fitting to which a shop-type electric compressor could be used to keep the tank(s) at pressure. If the distance were sufficient between the two, might make it a lot easier to find that leak.

 

[Wornout Wrench]

This is great information. Thank you!
None of the tank drains are open and my hearing is good...I think.
I like the idea if a remote fill for the system because the engine is so darn loud and if the leak is near the engine as you say, it would be impossible to hear.
I've got a friend coming by to help. I'll rport back with my findings.
Thanks again.
Andy

Any time, glad to help out.