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Old 10-21-2021, 01:19 PM   #1
Mini-Skoolie
 
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Air pressure is low

Hi guys!

Just recently when starting a bus I noticed a weird "ts-ts-ts..." sound coming from underneath, like air pushing out, and its near the compressor tank.


The air is low, and been around 50psi for long time and it seems not going lower.


I checked with my hand and I feel the air is pulsing out from that black spot in the middle of the second picture.


What could be the problem?


Bus is Thomas Freightliner 2007
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Old 10-21-2021, 01:32 PM   #2
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thats your air dryer.. if the pressure goes up to 120 then it goes SHHHH! for a second then stops.. and pressure holds up for a good while and then builds back up and goes SHHH.. thats normal.. if its never letting the pressure go up any higher before it goes SHHH then thats NOT normal..


thats the purge valve in the air dryer.. its designed to purge the moisture out of their when the governer tells the compressor to turn off (usually around 120-130)..


the governer also tells the air dryer to purge.. so you could have a bad air governor (little grey device wit ha round cap on the side or top of the compressor that has a cpuple small air lines going to it).. or the air dryer is malfunctioning...
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Old 10-21-2021, 02:05 PM   #3
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Thank you very much for the reply and explanation! Much appreciated!



Is it safe to drive like this?


Is there something I can do (test, connect-disconnect, lubricate, wind, hammer, etc..) to ease the problem?
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Old 10-21-2021, 02:11 PM   #4
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if the air pressure is staying that low you CANNOT drive it..
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Old 10-21-2021, 03:00 PM   #5
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Never drive with low air pressure, unless absolutely 100% necessary. It's dangerous, and can be very unpredictable.

As christopher said, that's the air dryer, and the purge valve in the dryer is the black spot that is leaking. The purge valve is supposed to open and close freely based off the governor. You have an AD9 air dryer for reference.

If the purge valve is stuck open, there are a few things you can do.

If you've never serviced your air dryer, you can buy yourself a dryer cartridge and purge valve rebuild kit and service it. There are videos on youtube that show how to do that and install both.

Or, you might be interested in installing a remanufactured dryer. Sometimes a dryer might be too corroded to service, which makes replacement necessary.

What you can do in the mean time is spray lubricant into the purge valve and tap on it to get it freed up again. Some have done that on here and got theirs working. But I'd highly recommended that you service or replace it, as that's something we do yearly on school buses as part of preventative maintenance.
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Old 10-21-2021, 03:58 PM   #6
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Thank you very much for the information!


I will look for the possibilities to maintain or replace it asap.

But while my bus is stored, is it ok to start it? Keep it running? Move it, back and forth, if necessary? Just for other maintenance.



As much as Im aware, I shouldnt have air breaks.
All I know that works on air is the stop sign and my drivers seat...
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Old 10-21-2021, 04:12 PM   #7
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In 2007 the Freightliner came with air brakes. If the pressure doesn't build to 65-75psi, it will not allow the brakes to be released to drive. There will be a warning buzzer until it reaches 65-75psi.
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Old 10-21-2021, 04:39 PM   #8
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Quote:
Originally Posted by kapa View Post
But while my bus is stored, is it ok to start it? Keep it running? Move it, back and forth, if necessary? Just for other maintenance.
Yup, no issues starting or running. It likely won't move as it is, and I wouldn't try to move it, without repairing it first. What other maintenance are you planning on doing? There's no need to start and run a bus periodically to "keep it good". Doing so actually does more harm. If storing long term, there's things you do for that. But don't start it up unless you plan on taking it somewhere.


Quote:
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As much as Im aware, I shouldnt have air breaks.
You *should* have air brakes. In 90+% of the world they're actually desired. A lot of your vehicles had hydraulic brakes years ago, and there's a reason we've gone away from that. Most people that I meet who dislike them do so because they don't understand them.
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Old 10-21-2021, 05:34 PM   #9
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Thank you again guys for helping to understand!


In a way, I really do wish I’d have air brakes, but my brakes look like on the picture. For me look like a typical hydraulic brake calipers with rotors. And they have a oil line going in. All four wheels.
Unless this oil line is pushed by air, but wouldn’t be logical, right?


And I only have the light on the dashboard without any peeping sound.

When I do press the brake pedal or operate the parking brake lever I dont get any air sound effects either.


Quote:
There's no need to start and run a bus periodically to "keep it good". Doing so actually does more harm. If storing long term, there's things you do for that. But don't start it up unless you plan on taking it somewhere.
The bus is in my backyard, now probably gonna be there for another cold winter.
But because I live in a cold environment I would think starting it periodically would be doing good? Keeping things moved and lubricated, fluids flowed, batteries charged, no?
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Old 10-21-2021, 05:43 PM   #10
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Starting a diesel is only good if you warm it fully up till the thermostat opens on the engine. Setting high idle at 1200-1400 after it has been started and run a minute or so.. if you don’t warm it fully then you do more harm than good by inviting condensation to occur on the cylinders and in the oil .

If it were mine is fix the air issue now before winter
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Old 10-21-2021, 05:44 PM   #11
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Interesting.

As that is a hydraulic brake caliper. What air accessories do you have on this? Air ride suspension or a kneeling feature? Are you in Canada or Alaska? As freeze ups in air systems are one of the few reasons to go with a hydraulic brake bus.

And no, it wouldn't do good to keep things moved and lubricated, fluids flowed, batteries charged.

Remove the batteries and keep them indoors and charged. Winterize fuel systems. Drain air tanks. Check coolant for correct freeze point. Cover tires to prevent UV rot.

Other then that, nothing else needs done.

Only thing you're doing by moving it is keeping the brakes from rusting. But most engine wear comes at startup, and not getting it fully warmed up by taking it on a trip will accumulate moisture and acidity in the oil, making it worse. It's better to let it sit.
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Old 10-21-2021, 05:49 PM   #12
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yup that is an unusual combination

Have an engine driven air compressor AND hydraulic brakes is not the normal.

I agree that is a hydraulic brake caliper...

The 50 psi might be normal for your particular system. For air brake system 50psi is too low.

Yes you can move your bus... I wonder if you have a parking brake that is air operated....

william
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Old 10-21-2021, 05:57 PM   #13
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Much obliged, guys!

Cadillackid, thats exactly how I do it. Thank you!


Booyah,

Yes I do live in Canada, Quebec. I was also thinking probably that is the reason for not having air brakes.


I have drivers seat, stop sign, i think also ride suspension but thats all i could think of.



And I do warm it fully up, until I get warm air from the heaters. But thank you very much for your suggestions! I do think its better idea to store batteries inside and leave it as it is.


I wonder now how can i empty the air?
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Old 10-21-2021, 06:10 PM   #14
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Quote:
Originally Posted by magnakansas View Post
Have an engine driven air compressor AND hydraulic brakes is not the normal.

I agree that is a hydraulic brake caliper...

The 50 psi might be normal for your particular system. For air brake system 50psi is too low.

Yes you can move your bus... I wonder if you have a parking brake that is air operated....

william



my Red Bus is Hydraulic Brake and has a noirnal bendix air compressor, governer, dryer (heater).. all for the air ride suspension and the air ride driver seat... nothing else is on it.. single Tank..


it pumps up to 120 cuts off.. then goes to just below 100 for cut in
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Old 10-21-2021, 06:11 PM   #15
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Quote:
Originally Posted by kapa View Post
Much obliged, guys!

Cadillackid, thats exactly how I do it. Thank you!


Booyah,

Yes I do live in Canada, Quebec. I was also thinking probably that is the reason for not having air brakes.


I have drivers seat, stop sign, i think also ride suspension but thats all i could think of.



And I do warm it fully up, until I get warm air from the heaters. But thank you very much for your suggestions! I do think its better idea to store batteries inside and leave it as it is.


I wonder now how can i empty the air?
There will be a release/drain valve on the tank. Air tanks should be drained before every use.
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Old 10-22-2021, 12:48 AM   #16
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cadillac, well

I learned something else new today....


I have this little air compressor for an air ride seat on a Thomas... But until today I have never heard of a big compressor system on a hydraulic brake set up.

Now that I take the time to think about things I have seen this. A 1988 UHaul, international S series with air suspension has hydraulic brakes and this big honkin air compressor.

william
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Old 10-22-2021, 08:06 AM   #17
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so much can go wrong with air systems that it might be better to find a mechanic to stop by and take a look.
compressors can wear out causing it to not only not create pressure but to pump oil into your air system. unloader valves can stick open. governors can fail or malfunction and limit it to low air. any 1 0f 50 plus air lines can leak. tanks can rust out and crack. fittings like to crack and leak. air driers can plug (as in excess oil) and blow off the safety valve. purge valves can wear out and malfunction. ect that list usually covers most issues. to diagnose it you should have some tools with a gauge and start at the compressor then follow the system til you find the issue. if you got lots of oil blow by in your compressor you can rebuild it just like a engine cylinder with a hone and new pistons/rings. if its a leak get a large (1 or 2 gallon) fill it with warm water with 2 cups of dish soap (do not skimp) spray everything compressor, air dryer, governor (please note that sometimes in older vehicles air governors were remotely mounted on the firewall) all air valves (relay q. r., treadle, air leveler, protection and even air door and seat. (these are the reasons i chose hydraulic disk brakes). before working on air read and learn. good luck and be suspicious of the internet
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Old 10-22-2021, 10:13 AM   #18
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Quote:
Originally Posted by kapa View Post
Yes I do live in Canada, Quebec. I was also thinking probably that is the reason for not having air brakes.

And I do warm it fully up, until I get warm air from the heaters. But thank you very much for your suggestions! I do think its better idea to store batteries inside and leave it as it is.


I wonder now how can i empty the air?
Yup, freeze ups can occur with air brakes, so if you're in a cold, and I mean frigid cold climate, juice brakes are still used.

Probably has a parking brake on the back of the trans. I don't think anything made in the last 30 years uses a lucas-girling system(thank god).

No amount of idling will fully warm the engine oil in my experience. So drive it, or leave it sit off.

To drain the tank, there should be a petcock, on the bottom. You can see the tank in your 2nd picture, and what looks like an a auto-drain valve too. Take another picture of the tank and we can point you to what to open.
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Old 10-22-2021, 11:10 AM   #19
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Originally Posted by Booyah45828 View Post

No amount of idling will fully warm the engine oil in my experience. So drive it, or leave it sit off.

To drain the tank, there should be a petcock, on the bottom. You can see the tank in your 2nd picture, and what looks like an a auto-drain valve too. Take another picture of the tank and we can point you to what to open.

I agree, starting a cold engine, letting it high idle until rad and heater are "hot" and then turning off are worse than not starting it.
You have to run it long enough to "burn off" the existing condensation. Which is difficult to determine running time required.
Starting and getting engine up to temp and then shutting down just adds another cycle of condensation production.

Many air tanks are multi chambered and will have more than one drain valve in the bottom.
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Old 10-22-2021, 01:27 PM   #20
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wow!! i never knew that letting an engine idle to warm up till the Tstat opened was a bad idea? ive been told for years thats how its done.. and the people that told me never had an engine die before 300k...



as for me i usually drive my busses quite a bit so I dont worry so much.. (the reason I own them is to drive them not park forever someplace)
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