Air gauges show full, parking brake doesn't pop, still low air

[Drewser]

Bus: 2003 RE Blue Bird All American, air brakes, comfort ride air suspension. There are 3 air tanks, 1 in back, one in middle front between wheels, and one under drivers seat. 2 gauges in dash, front and rear.

To start, I drive busses for the school and am familiar with air brakes. But I need some help understanding the full system so I can figure out what's going on with mine. We've fixed 80% of the leaks, mostly due to crappy air tank drain valves...put proper ones on with cords. However, still have a leak somewhere, and while I'll be crawling under with a bottle of soapy water soon, I need help understanding how this all works.

I can air up the bus and the front and rear tanks are holding air. Gauges show it, and brakes still work, showing air go down as I push them. Furthermore, I can leave it sit overnight, wheels chocked with parking brake in and the button never pops out, gauges still show air. However, by morning, the accessories don't work (air horn, door, drivers seat) and the tank under the driver's seat is empty. Front tank between the wheels will give a few spurts of air and then be empty. But again, both gauges show full and brakes work. If I start the bus, I get low air alarm until front tanks get past 75 (I'm assuming...no gauge), and when they meet the pressure on the gauges, the gauges start going up too as they all fill.

If I air everything up and shut it down, I can hear something in the back slowly letting out air and then it will stop. Then I'll get a quick blow-off, like when compressor shuts off. Then after about 5 minutes the rear airbags will start releasing and slowly drop down. Front and rear gauges will still show air.

Questions:
1. Is it normal for the comfort ride to drop itself down after bus is off for a bit, or is this to conserve air for the brakes due to a leak?

2. Is there some proportioning valve that will keep front and rear gauges showing full and running off rear tank, even when front tanks are empty, or is something set up wrong?

BTW, while it's my bus now, I drove this bus for the private school for years and it had similar symptoms. But since it always passed air brake test, we just let it fill and they never bothered troubleshooting. I'd like to figure out what's going on.

[Drewser]

In doing my research, this sounds like it might apply:

Pressure Protection Valve
The pressure protection valve is normally a closed
pressure sensitive control valve. These valves can
be used in many different applications but are
typically used to protect or isolate one reservoir from
another by closing automatically at a preset pressure.
The valve is also commonly used to delay the filling
of auxiliary reservoirs until a preset pressure is
achieved in the primary or braking reservoirs.
Pressure protection valves allow air to be "shared"
between two reservoirs above the closing setting of
the valve. The sharing ceases when pressure drops
below the closing pressure off valve and the
reservoirs are then isolated from each other.

Would explain why brakes still work, but I'm still surprised front tank gauge shows full pressure even when tank is empty...

[ewo1]

I would not rule out a sticking pressure gauge....

Not sure if your gonna find the answer in THIS THREAD but I did post up the air brakes section for an International/Amtran chassis. maybe there something there that might provide some insight..

[Drewser]

Originally Posted by ewo1

I would not rule out a sticking pressure gauge....

Honest question...if the gauge was sticking, is there a scenario it would still go down slowly when hitting brake pedal? I can see both tank gauges drop each time I hit the brakes with bus off even if front tank is empty. I would expect front to stay put or completely drop the first time I hit the brakes. It's like it's measuring air in the system instead of the tank.

Thanks for the thread, looking through it now.

[Drewser]

ewo1, that thread is good stuff. I know it's not my system, but a lot of these air systems are pretty darn similar. This section of Air brakes Chapter 6 - Inversion valve may apply:

Brake Application With Loss of Air in Secondary System
If air pressure is lost in the secondary reservoir, the primary [rear] reservoir as well as the spring brake control valve is protected against air loss through action of single check valve (air source to primary reservoir) and double check valve. A brake application at brake valve in this situation results in little or no air being delivered from the secondary system [front] to the control port of inversion valve. No movement of the internal components of the valve takes place. Braking is assured since the primary reservoir is protected and the primary delivery system of the brake valve will apply the service brake portion of the spring brake chambers.

This section in "Air Brakes Chapter 3 - VALVES-Check_protection-pressure" was also interesting:

PRESSURE PROTECTION VALVE
The pressure protection valve is designed to close off air supply to any air operated device as long as the air supply is below a specified rating [rating/pressure info here]. It is important that this valve be installed when any auxiliary attachments are installed on a vehicle to avoid complete loss of air in the event of an air leak in any of the air operated attachments.

This explains the horn, door, and seat not working but brakes still working. At least I can isolate the leak. Now time to climb under and see where the front pressure gauge taps into the system. And see if the front brakes actually engage when the front tank is empty.

By the way, I'm adding quotes just in case it helps anyone else later and to make it more searchable. I hope people don't mind me cluttering up the thread.

[ewo1]

Here is a link for the BB All American A3 (2003) braking system.
It's 4mb so I can't upload here.

https://www.centralstatesbus.com/wp-...s/a3brakes.pdf

[Ronnie]

On my bus it is one tank divided into three. The front tank does not feed the brakes, only the second and third do, and they are protected by one way valves, so if the first tank loses air there is still air in the second and third.

[Iceni John]

How come you have only three tanks? I thought four was normal, especially if you have air bags. I have a wet tank that feeds the two brake tanks (rear/primary and front/secondary), and the accessories tank (for suspension/seat/wipers/door/horn/etc), with a PPV that won't start to fill the accessories tank until the brake tanks have reached about 65 PSI. There should also be a check valve at the inlet to the wet tank, or it can be at the air dryer's outlet: if it leaks then weird things will happen.

It's very unlikely that both your air gauges are inaccurate or sticking - simple Bourdon-tube mechanical gauges are usually pretty reliable. You have a serious leak somewhere, so it's soapy-water time!

John

[s2mikon]

The air leveling valves are a constant source of leakage on all air ride buses, trucks and trailers. It is a high wear item. DO NOT get under your bus with out adequate jack stands holding it up. I have personally witnessed a air bag failure on a trailer while in the shop. It came down fast. No one was under it and the guy working on it was doing an oil change but it shook him up none the less. 2 pounds per minute leakage is your out of service limit. If you are leaking 60 pounds in an hour your fine. I would like to have 0 leakage but it seldom ever happens. Also check your water separator / filter dryer for leaks. They are re-buildable.

[Drewser]

Good stuff everyone, thanks! I'll keep chasing down the leaks. Will post back when I find something. Still weird to me that both gauges show full even when both front tanks are empty...

[Mekanic]

I too am a school bus driver. My friend at our bus garage has shown me that the buses have pressure regulators for the accessories (horn and seat that limit pressure to 60psi). It sounds like it maybe needs looked at.

[Drewser]

Originally Posted by Mekanic

I too am a school bus driver. My friend at our bus garage has shown me that the buses have pressure regulators for the accessories (horn and seat that limit pressure to 60psi). It sounds like it maybe needs looked at.

Thanks. Def needs more looks.

I'm still interested to hear if anyone has theories as to why both front and rear gauges can show full while the 2 front tanks are empty. And they are not stuck...like I mentioned, they both drop with each brake press and engine off. I've been looking through diagrams too but can't put my finger on it...assuming it has something to do with rear tank feeding front brakes when tank is low or there's a leak, but seems like front gauge should reflect accurate front tank, unless the rear tank is split and runs all brakes...

[Booyah45828]

Originally Posted by Drewser

Thanks. Def needs more looks.

I'm still interested to hear if anyone has theories as to why both front and rear gauges can show full while the 2 front tanks are empty. And they are not stuck...like I mentioned, they both drop with each brake press and engine off. I've been looking through diagrams too but can't put my finger on it...assuming it has something to do with rear tank feeding front brakes when tank is low or there's a leak, but seems like front gauge should reflect accurate front tank, unless the rear tank is split and runs all brakes...

Because your "front" tank is full, while the tanks up front are empty.

Let's start by calling things what they are. FYI it's best to call those gauges primary and secondary tanks. The primary tank feeds the rear axle, whereas the secondary tank feed the front. What you're referring to as front, is the secondary system that feeds the front axle brakes, not necessarily the tank that's up front.

*Anything on this side is typically red in color, primary is green in color. Sometimes manufacturers will use colored hose throughout as well. Your gauge on the dash might be faded to orange, as are the nylon lines. Sometimes they used orange to show secondary, but usually orange is reserved for parking system.

Now that that is covered.

Yes the gauge could be sticking, but more then likely your tanks that are up front are auxiliary tanks, and are feeding the auxiliary accessories(horn, seat, air ride suspension, etc), and their air pressure isn't represented by a gauge on your dash.

Pressure is built by the compressor, usually through a drier, and into a wet tank(who's pressure isn't displayed on the dash either). From the wet tank it gets fed through check valves into primary/secondary tanks, and then possibly a ppv valve into the auxiliary/accesory tanks.

The wet tank is out of air, probably the auxiliaries too, and thanks to the check valves, the primary/secondary tanks are still fine. Your air alarm is on the wet tank, which is empty and why it sounds, yet the gauges look fine.

Originally Posted by Drewser

There are 3 air tanks, 1 in back, one in middle front between wheels, and one under drivers seat.

My guess is that the 1 in back is the typically tri-tank found on school buses the middle one is for air suspension, and the one under the seat is for auxiliary air. That tri-tank would be your primary, secondary, and wet tank. Does it have 3 drain valves on it? That's an indicator that it's actually 3 tanks, and they're is 2 welded walls internal separating the 3 from one another. If you look close there should be an inch hose on top feeding the check valves and tanks.

My guess it the check valve between your drier and wet tank isn't working. That's why

Originally Posted by Drewser

I can hear something in the back slowly letting out air and then it will stop. Then I'll get a quick blow-off, like when compressor shuts off. Then after about 5 minutes the rear airbags will start releasing and slowly drop down. Front and rear gauges will still show air.

happens. Air pressure in the wet tank leaks backwards into and out of the drier, and when pressure drops low enough, the governor pulls air pressure from the compressor release and drier drain.

Bendix handbook on air systems, and their videos too, are invaluable to anyone working on air systems. Read and watch those and you'll get a better understanding of how the systems operate. There are a lot of redundancies in the systems for fail safe measures, don't bypass or disable any of them!

[Drewser]

Originally Posted by Booyah45828

Because your "front" tank is full, while the tanks up front are empty.

(snip)

Avoiding quoting the whole post, but this was exactly the kind of explanation I was looking for. Didn't realize my rear tank could be running both gauges. Everything you said makes total sense (though my rear tank only has 2 drains...as do both front tanks). Makes me want to leave work and crawl under the bus to verify everything and see what goes where to confirm...

Gauge hoses match what you are saying too ...

[Booyah45828]

Sometimes your wet tank will be a loner that then feeds a dual primary-secondary tank. I don't see that often, but have seen it before.

Most tanks that I see on buses are the tri-tank variety, and then they might add another tank as an "auxiliary" tank to run air volume heavy accessories, like air ride. The oldest buses we have in the fleet are an 04 BB AAFE and both of those have the tri-tank. So I can't help you with any examples.

Ewo1 was nice enough to link the manual for you. I'd check that out and follow to see if you can ID what you actually have. I don't have the time to open and look at it, and nothing in the fleet is that old anymore. If you can't figure it out from the manual, follow the line from the compressor to the drier, and then the drier to the first tank, which would be known as the wet tank. Even with a drier they will still have a wet tank to collect crud. From there it should feed into primary/secondary, and then through a ppv and into any auxiliary tanks.

I know you said you replaced the drains, but sometimes they'll put a drain on the side toward the bottom instead of on the bottom itself. Maybe a drain was overlooked?

[Drewser]

I'll definitely take a look at that back tank, and follow some lines. Another really helpful post. Thanks so much! Feeling more confident in uncovering what's up down there now.

And I have all those manuals downloaded and working my way through.

[Drewser]

Update: Crawled under the bus over the weekend. Rear tank and front mid-tank are both welded in the middle, so effectively 2 tanks each. We could hear what sounded like a check valve letting air from the front back.

So started at the front and worked our way back spraying connections and looking for bubbles. Didn't find any connections leaking. As we were laying under there, my son for kicks sprayed the air bags and bubbles went crazy. Time for new bags. But if those were leaking, it could explain why my front tanks were draining sending air back to the bags. Did also confirm that the one tank drain I didn't replace was also leaking. Fleet Pride only had 5 and I'd ordered the 6th on Amazon. So got that swapped out too.

Next step is to jack it up and replace the rear bags and see how things go.