Question about air brakes.

schooliefiend

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Joined
Jan 1, 2025
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17
So, I picked up a 1995 Thomas full sized school bus and I found the brakes to be difficult. On a car, you press the brake a little and it applies the brakes a little. With these brakes, it was almost either complete on or completely off. I really had to press the brake hard to get it to go anything and then it would make the stopping process really jerky. Is this normal for air brakes or do I need to have them adjusted somehow? Thanks!!
 
It sounds like at a very minimum, you should check your brake adjustments at the slack adjusters. You should also check your brake shoes to make sure they aren't excessively worn down.The slack adjusters and the brake components probably could stand a little grease, too (there are zerks there. You could also have an issue with the airlines. You may not know how to do any of this (like a DOT leak down check ...?) so I would get someone to do all of this for you. It's not terribly complicated but there is definitely a procedure to all of it. That's why we commercial drivers have to take an air brake test for our CDL's ;) Checking your air brakes is a critical part of the daily pre-trip inspection, too. You should get someone to show you how to do that. Even systems with Automatic Slack Adjusters need to be checked from time to time.

Remember, though -- the air brakes were probably adjusted at some point for best function at close to max weight. They should be a little touchy when the bus is empty. But properly adjusted brakes shouldn't need to be stood on to work. You *do* have an issue so its best to get it checked out before you drive it much more.
 
Thanks a bunch! I'll find some professionals to take a look at it since I don't understand much of what you just said. :)
 
So I have a 1995 Thomas, and I drove 200 miles on my purchase date of the bus, and I found that one of my front brake chambers, the big round thing the air line plugs into, the plunger was completely shot and there was no resistance to it, so it essentially wasn't even strong enough to pull the slack adjustor.

Your system works like this. Air line -> Service Brake Chamber -> Slack Adjustor -> Brake S Cam, -> To Drum springs, to brake pads.

All of those items need to work for your air brakes to work properly. If you have broken springs, your pads don't connect when your slack adjusters pull.

If your slack adjustor fails to pull, your S Cam never turns.

If your service brake chamber fails to work, your slack adjustor never sends pulls the S Cam.

My Slack adjusters were also shot, and the rubber spring that tightens the slack with was shot as well. So I myself replaced the Drum Springs, Service brake chamber, and slack adjusters. The S CAM usually does not go bad but it can, but rare.

I'd start with those 3. Have a professional look them over.
 
I used to drive a dump truck with air brakes and it was screeching the tires and trying to put you through the windshield or you thought you were going to rearend someone the, dealer adjusted the air brake master cylinder and it worked correctly
 
Properly maintained air brakes actually work pretty good and should be easy to apply smoothly without the grabbing you are describing. S cams need to rotate smoothly no sticking allowed, brake can diaphragms without holes and good shoe springs and hardware. A real plus is a compressor tank system that keeps pressure above 110 lbs running.
 
Stove bolt has a good idea. Learn the air brake check. As you drive more you will adjust to the different feel.
I drive a bus for my reg job. CDL drivers are required to do this test EVERY DAY. It is that important. Bus really don’t don’t have different brake than large truck. The contact area for drum type air brakes are Many Many times larger than most other vehicles.
 
Dont forget the treadle valve "foot valve" and then the relay valve and then the air chamber and then pushes the slack adjuster.
 
Stove bolt has a good idea. Learn the air brake check. As you drive more you will adjust to the different feel.
I drive a bus for my reg job. CDL drivers are required to do this test EVERY DAY. It is that important. Bus really don’t don’t have different brake than large truck. The contact area for drum type air brakes are Many Many times larger than most other vehicles.
There's a lot that goes into air brakes. But you really only need to check 2 things. Does it stop well, (if it's barely stopping you got a chamber issue or something else), and does the air maintain 90 PSI+ while running, and does a pump down test succeed.

Ok so 3 things, but if those 3 things are checked and working, you'll be fine to go out on the road. If there are any other issues with other items, one of those 3 things will fail, and you will need to inspect the whole system and not drive on it till it's corrected.
 

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